FIX: replaced temp internalField with DimensionedField::null (fixes #3082)

GeometricField::Boundary allocated using createZeroBoudnaryPtr held a dangling reference to a temporary object (see also #3082 (closed)).

This dangling reference was never accessed, so it didn't cause any wrong memory access. It did however increase the peak memory consumption.

Replaced the temporary object with DimensionedField<Type, volMesh>::null