[Bindings] Emit less code per member in generated dictionary ToV8.
Currently, we emit two invocations of CreateDataProperty for members
that are defaulted or nullable. Since the invocation and handling is
identical in all cases where we store a value, we can emit the code to
do so just once.
This reduces Android APK size by 16 KiB. The difference in specific
symbols is can be fairly dramatic, with code size of conversion methods
reduced by up to 35%.