Dont start mocha if there are no files

I use grunt-mocha-test in combination with an watch event, to only run against the changed files/tests. Therefore it can happen, that there are no associated tests for a given file.
In these cases grunt-mocha-test runs with no files, which is not necessary.

I've added a check against `this.filesSrc.length` to stop the task without error on an empty list.