fix: launcher kill method which was throwing an error if no callback was specified

bug introduced in https://github.com/karma-runner/karma/commit/8647266fd592fe245aaf2be964319d3026432e33

In server.js when singleRun = true, no callback is passed to when a browser complete to launcher.kill so if a browser is not yet connected but one has already finished the process throw an error.

TypeError: undefined is not a function
    at process._tickCallback (node.js:415:13)