Enum fields don't handle empty string

[timsavage]

At present, the enum field only handles None values. If enums are used with a codec that does not support None (eg CSV) then a validation error is raised if there is not an enum member with a value of an empty string.

Behaviour should be if an empty string is provided, attempt to convert to an enum value if that fails then check for an empty string as well as the null flag before raising a validation error.

[timsavage]

A workaround is to add an enum with an empty string value.