Correctly handle variadic instances with empty arguments

[ilevkivskyi]

Fixes #16199

It was surprisingly hard to fix, because all possible fixes strongly interfered with the code that makes "no-args" aliases possible:

l = list
x: l[int]  # OK, same as list[int]

So after all I re-organized (and actually simplified) that old code.

[ilevkivskyi]

Oh wow, it looks like all the errors in the primer are real. Previously we didn't report an error for missing type argument for a generic class if it appeared inside a union in a target of a type alias. I added a regression test.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

schema_salad (https://github.com/common-workflow-language/schema_salad)
+ mypy-stubs/mistune/block_parser.pyi:19: note: In module imported here,
+ mypy-stubs/mistune/markdown.pyi:4: note: ... from here,
+ schema_salad/makedoc.py:36: note: ... from here:
+ mypy-stubs/mistune/scanner.pyi:19:52: error: Missing type parameters for generic type "Match"  [type-arg]

cwltool (https://github.com/common-workflow-language/cwltool)
+ cwltool/job.py:115:65: error: Missing type parameters for generic type "partial"  [type-arg]

websockets (https://github.com/aaugustin/websockets)
+ src/websockets/typing.py:29: error: Missing type parameters for generic type "LoggerAdapter"  [type-arg]

python-chess (https://github.com/niklasf/python-chess)
+ chess/polyglot.py:14: error: Missing type parameters for generic type "PathLike"  [type-arg]

koda-validate (https://github.com/keithasaurus/koda-validate)
+ koda_validate/errors.py:164: error: Missing type parameters for generic type "PredicateErrs"  [type-arg]

streamlit (https://github.com/streamlit/streamlit)
+ lib/streamlit/type_util.py:237:41: error: Implicit generic "Any". Use "typing.Dict" and specify generic parameters  [type-arg]
+ lib/streamlit/type_util.py:237:47: error: Implicit generic "Any". Use "typing.List" and specify generic parameters  [type-arg]
+ lib/streamlit/type_util.py:237:53: error: Implicit generic "Any". Use "typing.Set" and specify generic parameters  [type-arg]
+ lib/streamlit/elements/arrow.py:49:5: error: Missing type parameters for generic type "Iterable"  [type-arg]