Improve empty collection handling with next

[max-muoto]

Fixes #12064.

[JelleZijlstra]

This doesn't seem right; it potentially leaves the type variable unsolved.

[github-actions]

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

werkzeug (https://github.com/pallets/werkzeug)
- tests/test_test.py:757: note:     def [_T, _VT] next(SupportsNext[_T], _VT, /) -> _T | _VT
+ tests/test_test.py:757: note:     def [_T] next(SupportsNext[_T], _T, /) -> _T

operator (https://github.com/canonical/operator)
- ops/main.py:194: error: Argument 2 to "next" has incompatible type "None"; expected "Storage"  [arg-type]
+ ops/main.py:194: error: Incompatible types in assignment (expression has type "Storage | None", variable has type "Storage")  [assignment]

ibis (https://github.com/ibis-project/ibis)
- ibis/expr/operations/udf.py:41: note:     def [_T, _VT] next(SupportsNext[_T], _VT, /) -> _T | _VT
+ ibis/expr/operations/udf.py:41: note:     def [_T] next(SupportsNext[_T], _T, /) -> _T

streamlit (https://github.com/streamlit/streamlit)
+ lib/streamlit/runtime/app_session.py:424:20: error: Argument 1 to "filter" has incompatible type "Callable[[Any], bool]"; expected "Callable[[str], TypeGuard[Optional[str]]]"  [arg-type]
+ lib/streamlit/runtime/app_session.py:424:36: error: Invalid index type "Optional[str]" for "Dict[str, Dict[str, str]]"; expected type "str"  [index]