validate: 'bat' is optional

If 'bat' is not installed G_IS_SUBPROCESS (subprocess) fails and the
error message is misleading.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8376>

subprojects/gst-devtools/validate/gst/validate/flow/gstvalidateflow.c

@@ -479,8 +479,11 @@ run_diff (const gchar * expected_file, const gchar * actual_file)
           "diff", "--paging", "never", "--color", colored ? "always" : "never",
           fname, NULL);
 
+      if (!error) {
         g_subprocess_communicate_utf8 (process2, NULL, NULL, &tmpstdout, NULL,
             &error);
+      }
+
       if (!error) {
         g_free (stdout_text);
         stdout_text = tmpstdout;