Bug 1024795, part 1 - Remove trailing whitespace from lower.py. r=bent
Bug 1024795, part 2 - static_cast isn't needed for nullptr in IPDL codegen. r=bent

Since bug 1038523, lower.py generates nullptr for NULL, so this cast isn't needed.
Bug 1024795, part 3 - Generate nsTArray instead of InfallibleTArray in lower.py. r=bent
Bug 1024795, part 4 - Use a default value for initializing unions, except for nsTArray. r=bent

This restores the initialization behavior prior to bug 819791, except for the one case that bug requires.