[MM-27836] Fix possible panic when patching config (#15262)

* Fix possible panic when updating/patching config

* Remove un-needed check

api4/config_test.go

@@ -550,6 +550,12 @@ func TestPatchConfig(t *testing.T) {
 		cfg, resp = th.SystemAdminClient.GetConfig()
 		CheckNoError(t, resp)
 		require.Equal(t, nonEmptyURL, *cfg.ServiceSettings.SiteURL)
+
+		// Check that sending an empty config returns an error.
+		_, resp = th.SystemAdminClient.PatchConfig(&model.Config{})
+		require.NotNil(t, resp.Error)
+		CheckBadRequestStatus(t, resp)
+		assert.Equal(t, "api.config.update_config.clear_siteurl.app_error", resp.Error.Id)
 	})
 }