By empty I think you mean an empty string that is not NULL? If that is the case, I think you need to ensure that all of those empty strings are converted to NULL prior to running this check. One or more space characters is probably being treated as just another text string that doesn't match a value in the coded value domain list. At least by converting them to NULLs, you have the toggle for "Search for Null Values" which you could use to ignore NULL values.
If your field does not allow NULL values, well, you're probably stuck adding a new value (to represent "unknown" or "not applicable") to both your data and the coded value domain.