Protect parsing of data types from empty lines

[lbetz]

If parsing empty rows in data type files the method first returns an nil class so strip doesn't work anymore.

[evgeni]

Thanks Lennart!

The rubocop failure is unrelated (fix in #376, which is merged, feel free to rebase on that).

Could you add a test for the empty line behaviour you're fixing?

[evgeni]

Applied suggestion and added a test.

[ekohl]

Btw, long term I'd still prefer to make this depend on puppet-strings to extract the actual data types instead of attempting to maintain yet another parser. Short term this is a good fix.