functionObjects::residuals: Writes "N/A" for fields not solved

Patch contributed by Bruno Santos
Resolves bug-report https://bugs.openfoam.org/view.php?id=2608

src/functionObjects/utilities/residuals/residualsTemplates.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2015-2016 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2015-2017 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -87,6 +87,10 @@ void Foam::functionObjects::residuals::writeResidual(const word& fieldName)
                 }
             }
         }
+        else
+        {
+            file() << token::TAB << "N/A";
+        }
     }
 }