Fix html reporter empty sourceStore problem with browserify

[johnny-mh]

i use this istanbul module for coverage reports.

karma-browserify + browserify-istanbul is powerful but has some issue.

the issue is sourceStore to be empty like below code.

console.log(sourceStore);    // { sourceCache: {} }
// this are make writeDetailPage() -> sourceText to undefined.

so i add some validate function for check empty sourceStore object.

[mwq27]

👍 I had this exact same problem. I added your changes and finally it's working. Thanks @everedifice

[yleviel]

I also tested this out and it fixed the html reporter errors for me.

[davglass]

Merged in 9c98c51 after a rebase with master to clean up the conflicts.