keeps plugin config on reenablement (#35545) (#35581)

* keeps plugin config on reenablement

* fixes local config patch on plugin reenablement

(cherry picked from commit c9a4092ac0a20351e3c2e0ac0cb593cc28b5bc0e)

Co-authored-by: Carlos Garcia <carlos.garcia@mattermost.com>

server/channels/api4/config.go

@@ -347,6 +347,11 @@ func patchConfig(c *Context, w http.ResponseWriter, r *http.Request) {
 		c.App.HandleMessageExportConfig(cfg, appCfg)
 	}
 
+	// Treating an empty plugins map as nil preserves the existing configs.
+	if len(cfg.PluginSettings.Plugins) == 0 {
+		cfg.PluginSettings.Plugins = nil
+	}
+
 	updatedCfg, err := config.Merge(appCfg, cfg, &utils.MergeConfig{
 		StructFieldFilter: filterFn,
 	})