If current package folder is null don't delete

[AndrewJack]

Fixes: #795

I've seen this in my crashlytics logs - same as the above issue.

Issue: getCurrentPackageFolderPath() can return null

Solution: check if path is null before deleting the directory.

[msftclas]

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[max-mironov]

Good catch and thank you for contribution @AndrewJack!
LGTM!
Also not a huge deal but but what do you think of moving this check inside deleteDirectoryAtPath method (also can add CodePushUtils.log stating that there was attempt to delete null path)? I've looked briefly through other calls to this method and looks like we don't have any issues with this currently but may be useful to have this check here somewhere in future? Anyway its up to you and I'm good with both decisions and thanks again for helping us to make the project better!

[AndrewJack]

@matrosov-nikita I've made those suggested changes