Links category can be set to blank

[SteveAgl]

A users signaled he was able (by mistake) to edita a link category, setting its name to nothing and saving it. Now the category show as blank in categories list without the checkbox and with any chance to change it in any way.

I tested it personally and it's true, I didn't check if post categories act in the same way.

Post categories aren't affected (the name is checked in wp_insert_category) but tags are. Not sure whether a fix would be possible in wp_upgrade_term or before that is called otherwise I'd have attempted one.

Patch adds in simple checking of $name in wp_update_term(). Returns WP_Error if empty.

(In [8393]) Don't allow empty term names. Props scohoust. fixes #7336 for trunk

(In [8394]) Don't allow empty term names. Props scohoust. fixes #7336 for 2.6