core: fix buffer overflow in function utf8_next_char and return NULL for empty string

Now the function utf8_next_char with an empty string returns NULL instead of
the next char, which is most of the time after an allocated buffer.

And the function utf8_char_size with an empty string now returns 0 instead of
1.

This indirectly fixes a buffer overflow in function eval_string_range_chars
when the input string is empty (for example when doing `/eval -n ${chars:}`).

src/core/hook/hook-command-run.c

@@ -115,7 +115,7 @@ hook_command_run_exec (struct t_gui_buffer *buffer, const char *command)
     if (command[0] != '/')
     {
         ptr_string = utf8_next_char (command);
-        if (string_asprintf (&command2, "/%s", ptr_string) < 0)
+        if (string_asprintf (&command2, "/%s", (ptr_string) ? ptr_string : "") < 0)
             return WEECHAT_RC_ERROR;
     }
     else