2.1 RC1 - Can't blank out a custom profile field's value

[MarkRH]

If I make a custom profile field and give it some value, I can not go back and delete the value to blank it out.  I can only change it to some other non-blank value.  I have to manually edit the value out in the smf_themes table.

[MarkRH]

Created GitHub issue: https://github.com/SimpleMachines/SMF2.1/issues/5483

[live627]

https://github.com/SimpleMachines/SMF2.1/pull/5522