Add nil check for user type in gen_app

[tchssk]

Fix #1060.

[tchssk]

Would you try this to your project? @luna-duclos

[luna-duclos]

Just tested this, code looks good, and code works! 👍

[raphael]

Thank you for the great PR with additional tests! I am wondering though if it wouldn't be better to put the test in the caller (https://github.com/goadesign/goa/blob/master/goagen/codegen/publicizer.go#L130) rather than in the Publicize function itself. Publicize should not be called on nil and protecting against that could hide other bugs (where the target should not have been nil but was because of a bug).

[luna-duclos]

I think a function not blowing up on nil is generally good behaviour. I can see your point about masking other bugs however, but considering these bugs would usually be the same nil check .. I don't see the harm

[raphael]

For example you may end up with code that is generated fine but does not compile because the Publicize method is missing. This is a lot harder to track than a clear error such as the one you hit where it took me 10 seconds to find the offending line. In general I dislike the idea of writing defensive code "just because" - it makes tracking issues down the line orders of magnitude more difficult.

[tchssk]

I reverted the commit and moved nil check to publicizer for hash. Is it correct?

[raphael]

Yes perfect, thank you for making the change!