Terminal: Revert "Simplify CLICOLOR_FORCE check"

Revert commit bbf6cdef (Terminal: Simplify CLICOLOR_FORCE check,
2021-05-03).  If `CLICOLOR_FORCE` is set to an empty string, it
will compare `!= "0"` but we still do not want to force color on.

Terminal.c

@@ -167,7 +167,8 @@ static int kwsysTerminalStreamIsVT100(FILE* stream, int default_vt100,
   /* Force color according to http://bixense.com/clicolors/ convention.  */
   {
     const char* clicolor_force = getenv("CLICOLOR_FORCE");
-    if (clicolor_force && strcmp(clicolor_force, "0") != 0) {
+    if (clicolor_force && *clicolor_force &&
+        strcmp(clicolor_force, "0") != 0) {
       return 1;
     }
   }