util: clean-ups to enum parsers

Never log when we fail due to OOM when translating enums, let the caller
do that. Translating basic types like enums should be something where
the caller logs, not the translatior functions.

Return -1 when NULL is passed to all enum parser functions.

The non-fallback versions of the enum translator calls already handle
NULL as failure, instead of hitting an assert, and we should do this
here, too.