fvcAverage and fvcReconstruct: Return zero for 0-D cases

fvcAverage and fvcReconstruct both do divisions or inverses of surface
summed fields. A single-cell zero-dimension case, has no genuine faces
on which to sum, so surface sums are identically zero. This change
detects this situation and returns a zero value instead of failing due
to a divide by zero.

This allows the multiphase test cases to be reduced to just one cell.

test/multiphase/reactingMultiphaseEulerFoam/populationBalanceModeling/binaryBreakup/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges

test/multiphase/reactingMultiphaseEulerFoam/populationBalanceModeling/breakup/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges

test/multiphase/reactingMultiphaseEulerFoam/populationBalanceModeling/coalescence/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges

test/multiphase/reactingMultiphaseEulerFoam/populationBalanceModeling/simultaneousCoalescenceAndBreakup/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges

test/multiphase/reactingTwoPhaseEulerFoam/populationBalanceModeling/drift/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges

test/multiphase/reactingTwoPhaseEulerFoam/populationBalanceModeling/negativeDrift/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges