fix: cast NexusNonNullDef to prevent union type complexity error

[tgriesser]

Ran into an issue around the generated return type for nonNull(), which produces:

NexusNonNullDef<any> | (TypeName & GraphQLNonNull<any>)

This is problematic when you reach a decent amount of types, where you'll hit a union type that is too complex to represent error. Removing the right hand side of the clause resolves the issue, and the fact that it's a GraphQLNonNull type is irrelevant, so we can just cast it to NexusNonNullDef<any> here

[codecov]

Codecov Report

Merging #846 (0435794) into main (d220d31) will not change coverage.
The diff coverage is 100.00%.

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  Coverage   93.91%   93.91%           
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  Files          46       46           
  Lines        2959     2959           
  Branches      695      695           
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  Hits         2779     2779           
  Misses        178      178           
  Partials        2        2           

[jasonkuhrt]

Looks like an easy thing to regress. Maybe a comment if not a test. LGTM!