Fix NullReferenceException in hover for generic methods returning a TypeVar

[idanbd]

Problem

Hovering a call to a generic method whose return type is a bare TypeVar (e.g. def get(self) -> T on a Generic[T] class), where the type parameter resolves to a collection type, throws a NullReferenceException while the server builds the return-type documentation:

at ArgumentSetExtensions.Values[T](IArgumentSet args)
at PythonClassType.CreateInstance(IArgumentSet args)
at PythonFunctionOverload.CreateSpecificReturnFromTypeVar(...)
at PythonFunctionOverload.GetSpecificReturnType(...)
at PythonFunctionOverload.GetReturnDocumentation(IPythonType self)
at HoverSource.GetHover(...)

Cause

GetReturnDocumentation calls GetSpecificReturnType(self, args: null) — there are no call arguments when producing documentation. For a TypeVar return on a generic instance this reaches CreateSpecificReturnFromTypeVar, which resolves the type parameter and calls specificType.CreateInstance(null). For the list/dict/tuple specializations, CreateInstance dereferences the null argument set (ArgumentSetExtensions.Values<T> / CreateList) and throws.

Fix

Guard PythonClassType.CreateInstance against a null argument set, returning a plain instance of the type. The normal call-resolution path (non-null args) is unchanged, and non-collection type parameters were already unaffected (they fall through to the same new PythonInstance(this)).

Test

Adds HoverTests.GenericMethodReturningTypeVar: hovering a Generic[T] method returning T resolved to a collection type no longer throws and returns the method's signature. (Complements the existing HoverGenericClass, which covers a non-collection type parameter.)