Select type form elements can silently fail validation

Select type form elements have had a few new things added to them, specifically #140783: A select list without #default_value always passes form validation. In doing this we've created a method by which select forms can fail validation completely silently, and the page just appears to reload over and over w/o ever actually processing.

This is done by not setting a default_value, which will make the #required value default to TRUE, and if a title attribute is not applied to the form element, then no error class is applied, and no message is received. This is because despite the validation failing, the check is to see if the error message has anything in it (which, it's an empty string, but an error was still set). As such we should check against NULL instead. Patch to follow.