update_usermeta should allow blank values

[jfarthing84]

I think it would be better if update_usermeta was allowed to insert '0' or blank values, instead of deleting them. First of all, if I wanted to delete the value, I would have called delete_usermeta. Second of all, update_post_meta allows it, so why not update_usermeta?

Sorry, I didn't realize on the first attachment that my code editor had converted the tabs from 6 spaces to 4, so it DIFFed the whole file. I uploaded the correct version. Instead of checking the meta value with the empty() function, just check for an empty string, that way the user meta value can be set to 0 if need be.