Explicitly test for the empty string in Channel_UserHasMode()

Explicitly test for the empty string in Channel_UserHasMode()

Basically this is unnecessary, as Channel_UserModes() always returns a
valid pointer and strchr() can deal with an empty (NULL-terminated)
string perfectly fine, bit it makes the code a bit more obvious and
silences the following warning:

  In function ‘Channel_UserHasMode’,
      inlined from ‘Channel_Kick’ at channel.c:384:7:
  channel.c:784:16: warning: ‘strchr’ reading 1 or more bytes from a region
                    of size 0 [-Wstringop-overread]
    784 |         return strchr(Channel_UserModes(Chan, Client), Mode) != NULL;
        |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This was seen with "gcc (Debian 12.2.0-14) 12.2.0" at least.