NULLIF not working for enum values

[amckinney]

I was exploring workarounds related to #374, and attempted to use the NULLIF function for an enum in my SELECT query, but noticed it wasn't working according to my expectation.

I created a reproducible example in a playground. In this case, I'm attempting to use NULLIF so that I can handle nil values for the role. Instead, the generated code turns the expected value for a role into a bool.

[kyleconroy]

Duplicate of #133