Check if child node is Q instance for subquery expressions

[CelestialGuru]

Fixes bug when using Exists subqueries which are or-ed together:

SomeModel.objects.filter(
   Exists(RelatedModelA.objects.filter(some_model=OuterRef("pk"))
   | Exists(RelatedModelB.objects.filter(some_model=OuterRef("pk"))
)

This is done usually for performance reasons, but it is a valid queryset. Exist instances ored together like this creates a Q instance, but the children are not themselves Q instances. They are Exists instances.

[j-antunes]

Thanks for opening a PR. Could you add the following:

• A unit test to cover your changes
• In your PR you change else to an elif, could you add another else so that in case none of the conditions are met, it will raise an exception or return something else.
  Thank you

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.35%. Comparing base (0d820d0) to head (acf288a).

Files with missing lines	Patch %	Lines
src/polymorphic/query_translate.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #572      +/-   ##
==========================================
- Coverage   75.40%   75.35%   -0.06%     
==========================================
  Files          21       21              
  Lines        1342     1343       +1     
  Branches      211      212       +1     
==========================================
  Hits         1012     1012              
  Misses        257      257              
- Partials       73       74       +1     

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