fix(enum-utils): return nil if option name is blank

[difernandez]

Context

In #442 we added support for automatic enum translation for Rails built-in enums

Changes

This PR fixes an issue that would show the whole enum hash as the translation for an empty value when using tag_column or tag_row. Thanks to @stadia for reporting this issue.
The fix returns nil as the translation for a blank value. Returning nil instead of an empty string keeps the same behavior as before the translation changes, which uses activeadmin's default translation for unset value

Before translation changes

After translation changes, before fix

After fix

I'm not adding this to the changelog, as it fixes something that is still unreleased.
I also added a test case for enum utils for when translate_enum_option is used with not nil value that is not present in enum, to account for all three cases.