Added a null check Fix issue #24

[michelemilesi]

Fixed the issue #24 "NPE on RegexRowRule when a row field is empty"
adding a null check and test case

[jeremybeard]

@michelemilesi, thanks for raising the issue and sending over the PR to solve it. Currently the project isn't ready for public contributions so for now we are developing it internally at Cloudera and just push releases to this repository. Hopefully that will change in the near future so in the meantime I'll leave this open, though there's a chance it might get fixed first as part of our internal work.

[michelemilesi]

@jeremybeard first of all thanks for your work.
I know, from Cloudera forum, that Envelope was closed for contributions (at the moment) and it sounds good for me.
If it is OK for I will open other issues and, if I can, add PR solving the problem.
I'm using Envelope in a couple of projects - on the biggest following a cloudera suggestion.

BTW it is OK for me if you will close or drop my PRs.