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.