Fixed error thrown when converting collapsed range

[mlewand]

The problem was caused by returning undefined instead of range instance when VSCode couldn't pick any word at given position.

Change in text returned in getSelectedText when there is no range is not necessary for this fix, but it will make the code more bulletproof in future.

As mentioned in #13 it looks also to fix #12, although I might be missing something in terms of error reproduction.

And by the way, thanks for nice extension! Definitely saved me some time.

[mlewand]

@wmaurer any chances on getting it in? Shouldn't be hard for you to review, while will reduce exceptions thrown by the extension. Thanks!