This change fixes the command arc look remotes for PHP 8.1.
Without this change, the null value bubbles up to PhutilUTF8StringTruncator, reaching a strlen().
This control probably does not need to be done at this low level inside PhutilUTF8StringTruncator,
but it is right to be at this high level from the caller in ArcanistRefView.
Closes T15368
✅ I verified that the above line is just null or a string
The function phutil_nonempty_string() will report any alien type, and that is OK.