Fixed case where plugin upgrade checker crashed

... if no extraneous plugins were installed. Hooray for not
testing the null case!

nodebb

@@ -118,8 +118,11 @@ var getRunningPid = function(callback) {
 				version: async.apply(getCurrentVersion)
 			}),
 			function(payload, next) {
-				var toCheck = Object.keys(payload.plugins);
+				if (!payload.plugins.length) {
+					return next(null, []);	// no extraneous plugins installed
+				}
 
+				var toCheck = Object.keys(payload.plugins);
 				request({
 					method: 'GET',
 					url: 'https://packages.nodebb.org/api/v1/suggest?version=' + payload.version + '&package[]=' + toCheck.join('&package[]='),