loader: Ignore empty override strings

If you somehow have MESA_LOADER_DRIVER_OVERRIDE= in your environment,
you certainly weren't trying to force load the driver named "".

Signed-off-by: Rob Clark <rob.clark@oss.qualcomm.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38165>

src/loader/loader.c

@@ -766,7 +766,7 @@ loader_get_driver_for_fd(int fd)
     */
    if (__normal_user()) {
       const char *override = os_get_option("MESA_LOADER_DRIVER_OVERRIDE");
-      if (override)
+      if (override && strlen(override))
          return strdup(override);
    }