fix(discriminator): union nullable

[karatakis]

Summary:
See the bug report #2981

Issue Reference(s):
Fixes #2981

Build & Testing:

• I ran cargo test successfully.
• I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• I have added relevant unit & integration tests.
• I have updated the documentation accordingly.
• I have performed a self-review of my code.
• PR follows the naming convention of <type>(<optional scope>): <title>

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.56%. Comparing base (3bdf43b) to head (84c8e8b).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2982   +/-   ##
=======================================
  Coverage   87.56%   87.56%           
=======================================
  Files         275      275           
  Lines       27253    27259    +6     
=======================================
+ Hits        23863    23870    +7     
+ Misses       3390     3389    -1     

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[meskill]

let's update the test with list case as well

type Query {
  node: Node @expr(body: null)
  nodes: [Node] @expr(body: [{id: 1, username: "user"}, null, {id: 2, slug: "page"}])
}

[meskill]

I think it could hide some errors when we are trying to resolve object type but getting something else. Null is a special case though, what about return Option<&str> here and for null use None?

[karatakis]

Closing because it is covered from the refactor here #3000