Fix null reference exception when extracting plural strings

[rjpowers10]

Fixes #72

[hishamco]

Could you please rebase on this branch https://github.com/OrchardCoreContrib/OrchardCoreContrib.PoExtractor/tree/FixPluralStringExtractor

[hishamco]

I did it from my side, just review the changes, let me know if they looks good to you

[rjpowers10]

Nice! I like the changes you made to the tests. Much better than what I had!

[hishamco]

Thanks for your contribution @rjpowers10, one more thing is there anything else we might need to add, coz I'd like to ship a patch release 1.0.1

[rjpowers10]

Thanks for your contribution @rjpowers10, one more thing is there anything else we might need to add, coz I'd like to ship a patch release 1.0.1

At this moment, no, I think this is good for now.

[hishamco]

FYI https://www.nuget.org/packages/OrchardCoreContrib.PoExtractor/1.0.1