make sure conversions of string-constant types properly handle empty strings

[ublubu]

In most (all? nearly all?) cases, it's valid for one of the type-alias-string fields to be empty.
(by type-alias-string, i mean e.g. type PodConditionStatus string)

Make sure the conversion functions don't treat empty strings as a failure case.

Also, be sure to add test cases where fixes are made.