BUG: #include of empty filename creates confusing error message (Mark Olesen)

When using #include or #includeIfPresent, it is easy to generate an
empty file name. For example,

    #include "~OpenFOAM/missing-path/file"

On error, findEtcFile() returns an empty fileName. This was treated as
a relative name, which meant there was an attempt to open the parent directory
as a file. The resulting error message was confusing:

    --> FOAM FATAL IO ERROR:
    Attempt to put back onto bad stream

    file: /CASE/PATH/system at line 1.

        From function void Istream::putBack(const token&)
        in file db/IOstreams/IOstreams/Istream.C at line 34.

- fix by leaving empty expansions as-is

src/OpenFOAM/db/dictionary/functionEntries/includeEntry/includeEntry.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2012 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -74,13 +74,15 @@ Foam::fileName Foam::functionEntries::includeEntry::includeFileName
     fileName fName(is);
     fName.expand();
 
-    // relative name
-    if (!fName.isAbsolute())
+    if (fName.empty() || fName.isAbsolute())
     {
-        fName = fileName(is.name()).path()/fName;
+        return fName;
+    }
+    else
+    {
+        // relative name
+        return fileName(is.name()).path()/fName;
     }
-
-    return fName;
 }