tracerutils: Do not warn on empty string as tracername

It doesn't matter if there is an "empty tracer" specified.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8860>

subprojects/gstreamer/gst/gsttracerutils.c

@@ -298,7 +298,7 @@ _priv_gst_tracing_init (void)
         } else {
           g_warning ("loading plugin containing feature %s failed!", t[i]);
         }
-      } else {
+      } else if (t[i][0] != '\0') {
         g_warning ("no tracer named '%s'", t[i]);
       }
       i++;