BUG: blockMesh mergePatchPairs fails with edge-shared points (fixes #2589)

- remedy by performing the attach() action sequentially (as per
  stitchMesh changes). This ensures that the current point addressing
  is always used and avoids references to the already-merged points
  (which is what causes the failure).

ENH: improve handling of empty patch removal

- only remove empty *merged* patches, but leave any other empty
  patches untouched since they may intentional placeholders for other
  parts of a workflow.

- remove any empty point/face zones created for patch merging

tutorials/mesh/blockMesh/mergePairs/Allclean

@@ -0,0 +1,10 @@
+#!/bin/sh
+cd "${0%/*}" || exit                                # Run from this directory
+. ${WM_PROJECT_DIR:?}/bin/tools/CleanFunctions      # Tutorial clean functions
+#------------------------------------------------------------------------------
+
+cleanCase0
+
+rm -rf constant
+
+#------------------------------------------------------------------------------

tutorials/mesh/blockMesh/mergePairs/Allrun

@@ -0,0 +1,8 @@
+#!/bin/sh
+cd "${0%/*}" || exit                                # Run from this directory
+. ${WM_PROJECT_DIR:?}/bin/tools/RunFunctions        # Tutorial run functions
+#------------------------------------------------------------------------------
+
+runApplication blockMesh
+
+#------------------------------------------------------------------------------

tutorials/mesh/blockMesh/mergePairs/system/blockMeshDict

@@ -0,0 +1,119 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v2212                                 |
+|   \\  /    A nd           | Website:  www.openfoam.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    object      blockMeshDict;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+scale   0.001;
+
+vertices
+(
+    // Block d
+    ( 50 -17 -12.5)
+    (100 -17 -12.5)
+    (100  -2 -12.5)
+    ( 50  -2 -12.5)
+    ( 50 -17  12.5)
+    (100 -17  12.5)
+    (100  -2  12.5)
+    ( 50  -2  12.5)
+
+    // Block e
+    (  0 -17  -12.5)
+    ( 50 -17  -12.5)
+    ( 50  -2  -12.5)
+    (  0  -2  -12.5)
+    (  0 -17   12.5)
+    ( 50 -17   12.5)
+    ( 50  -2   12.5)
+    (  0  -2   12.5)
+
+    // Block f
+    (  0 -19 -12.5)
+    ( 50 -19 -12.5)
+    ( 50 -17 -12.5)
+    (  0 -17 -12.5)
+    (  0 -19  12.5)
+    ( 50 -19  12.5)
+    ( 50 -17  12.5)
+    (  0 -17  12.5)
+);
+
+blocks
+(
+    // Block d
+    hex (0 1 2 3 4 5 6 7) block-d (4 5 4) grading (1 1 1)
+
+    // Block e
+    hex (8 9 10 11 12 13 14 15) block-e (4 4 4) grading (1 1 1)
+
+    // Block f
+    hex (16 17 18 19 20 21 22 23) block-f (5 4 4) grading (1 1 1)
+);
+
+
+edges
+(
+);
+
+
+boundary
+(
+    mid6
+    {
+        type patch;
+        faces
+        (
+            (0 0) // Block d
+        );
+    }
+    mid7
+    {
+        type patch;
+        faces
+        (
+            (1 1) // Block e
+        );
+    }
+    mid8
+    {
+        type patch;
+        faces
+        (
+            (1 2) // Block e
+        );
+    }
+    mid9
+    {
+        type patch;
+        faces
+        (
+            (2 3) // Block e
+        );
+    }
+
+    other
+    {
+        type  patch;
+        faces ();
+    }
+);
+
+
+mergePatchPairs
+(
+    (mid6 mid7)
+    (mid8 mid9)
+);
+
+// ************************************************************************* //

tutorials/mesh/blockMesh/mergePairs/system/controlDict

@@ -0,0 +1,48 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v2206                                 |
+|   \\  /    A nd           | Website:  www.openfoam.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    object      controlDict;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+application     blockMesh;
+
+startFrom       startTime;
+
+startTime       0;
+
+stopAt          endTime;
+
+endTime         0;
+
+deltaT          0;
+
+writeControl    timeStep;
+
+writeInterval   1;
+
+purgeWrite      0;
+
+writeFormat     ascii;
+
+writePrecision  6;
+
+writeCompression off;
+
+timeFormat      general;
+
+timePrecision   6;
+
+runTimeModifiable true;
+
+
+// ************************************************************************* //