STYLE: simpler construct for empty tmp field in volFieldValue

- originally (incorrectly) was a Field(0.0), which was generalized
  to Field(Zero), but Field() is the correct form

STYLE: rename 'mustGet' to more standard 'mandatory' variable

src/functionObjects/field/fieldValues/volFieldValue/volFieldValue.H

@@ -233,7 +233,7 @@ protected:
         //  Checks for availability on any processor.
         inline bool canWeight(const scalarField& weightField) const;
 
-        //- Return true if the field name is valid
+        //- True if the field name is valid (exists, and a supported type)
         template<class Type>
         bool validField(const word& fieldName) const;
 
@@ -242,7 +242,7 @@ protected:
         tmp<Field<Type>> getFieldValues
         (
             const word& fieldName,
-            const bool mustGet = false
+            const bool mandatory = false
         ) const;
 
         //- Apply the 'operation' to the values

src/functionObjects/field/fieldValues/volFieldValue/volFieldValueTemplates.C

@@ -40,16 +40,11 @@ bool Foam::functionObjects::fieldValues::volFieldValue::validField
     typedef GeometricField<Type, fvPatchField, volMesh> VolFieldType;
     typedef typename VolFieldType::Internal IntVolFieldType;
 
-    if
+    return
     (
         obr_.foundObject<VolFieldType>(fieldName)
      || obr_.foundObject<IntVolFieldType>(fieldName)
-    )
-    {
-        return true;
-    }
-
-    return false;
+    );
 }
 
 
@@ -58,7 +53,7 @@ Foam::tmp<Foam::Field<Type>>
 Foam::functionObjects::fieldValues::volFieldValue::getFieldValues
 (
     const word& fieldName,
-    const bool mustGet
+    const bool mandatory
 ) const
 {
     typedef GeometricField<Type, fvPatchField, volMesh> VolFieldType;
@@ -73,14 +68,14 @@ Foam::functionObjects::fieldValues::volFieldValue::getFieldValues
         return filterField(obr_.lookupObject<IntVolFieldType>(fieldName));
     }
 
-    if (mustGet)
+    if (mandatory)
     {
         FatalErrorInFunction
             << "Field " << fieldName << " not found in database"
             << abort(FatalError);
     }
 
-    return tmp<Field<Type>>::New(Zero);
+    return tmp<Field<Type>>::New();
 }