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

* Fix possible panic when updating/patching config

* Remove un-needed check

api4/config.go

@@ -180,7 +180,7 @@ func patchConfig(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	appCfg := c.App.Config()
-	if *appCfg.ServiceSettings.SiteURL != "" && *cfg.ServiceSettings.SiteURL == "" {
+	if *appCfg.ServiceSettings.SiteURL != "" && (cfg.ServiceSettings.SiteURL == nil || *cfg.ServiceSettings.SiteURL == "") {
 		c.Err = model.NewAppError("patchConfig", "api.config.update_config.clear_siteurl.app_error", nil, "", http.StatusBadRequest)
 		return
 	}

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)
 	})
 }