Bug fixes & String concatenation performance improvent

[nginth]

I ran into a few bugs when using ApexDoc for my org. I mainly fixed a NPE when @group-content points to a file that doesn't exist. I also included some (maybe not necessary) performance improvements by changing some string concatenation to StringBuilder appends.

[judisohn]

Thanks @nginth! Our development team is actively working on other projects so it may be a while before this can be reviewed and considered. We appreciate your support and patience!

[no-stack-dub-sack]

@nginth I realize this PR is quite old, but I'm attempting to revitalize this project here (I've yet to merge in my first big update, but there's on open PR for it - includes a few new tokens and some other new features), and I noticed this bug and then this PR in my travels. Are you interested in submitting this PR to the new repo once I merge the open PR? If not, I'd be happy to implement the main fix myself and attribute the credit to you. Thanks.

[nginth]

@no-stack-dub-sack I would be interested in submitting the new PR! I'll subscribe to your open PR and work on getting this one submitted over there when you merge it.

[no-stack-dub-sack]

@nginth Wow! That was a quick response 😄 There are some pretty significant changes with this first commit, so you likely won't be able to just copy/paste, but some of the sections you edited should be mostly the same. Merging this PR in shortly, see you over there!