Fix an error when fn===undefined (#3580)

* Fix an error when fn===undefined

This change fixes an error that occurs when reporter is 'xunit' and run is called without callback: " /rbd/pnpm-volume/a49b9a7c-c3f0-42ba-ae66-612ead86e96c/node_modules/.registry.npmjs.org/mocha/5.2.0/node_modules/mocha/lib/reporters/xunit.js:126
 fn(failures);  ^
TypeError: fn is not a function "

* Add test `.reporter("xunit").run(fn)`

This test has `try catch` block to catch an error if it occurs and write it to the console.

* formatting

* Update mocha.spec.js

lib/mocha.js

@@ -776,10 +776,11 @@ Mocha.prototype.run = function(fn) {
   exports.reporters.Base.hideDiff = options.hideDiff;
 
   function done(failures) {
+    fn = fn || function fn() {};
     if (reporter.done) {
       reporter.done(failures, fn);
     } else {
-      fn && fn(failures);
+      fn(failures);
     }
   }
 

test/unit/mocha.spec.js

@@ -56,6 +56,20 @@ describe('Mocha', function() {
     });
   });
 
+  describe('.reporter("xunit").run(fn)', function() {
+    it('should not raise errors if callback was not provided', function() {
+      var mocha = new Mocha();
+      expect(function() {
+        try {
+          mocha.reporter('xunit').run();
+        } catch (e) {
+          console.log(e);
+          expect.fail(e.message);
+        }
+      }, 'not to throw');
+    });
+  });
+
   describe('.addFile()', function() {
     it('should add the given file to the files array', function() {
       var mocha = new Mocha(opts);