Fix get_member_expr_fullname returning strings with embedded "None"

[brianschubert]

Fixes #17847

Before

from mypy.nodes import CallExpr, MemberExpr, NameExpr, get_member_expr_fullname
m3 = MemberExpr(MemberExpr(CallExpr(NameExpr("a"), [], [], []), "b"), "c")  # a().b.c

>>> get_member_expr_fullname(m3)
'None.c'

After

from mypy.nodes import CallExpr, MemberExpr, NameExpr, get_member_expr_fullname
m3 = MemberExpr(MemberExpr(CallExpr(NameExpr("a"), [], [], []), "b"), "c")  # a().b.c

>>> get_member_expr_fullname(m3) is None
True

[brianschubert]

@hauntsaninja py313-dev-ubuntu test failure:

https://github.com/python/mypy/actions/runs/11085498771/job/30801981409?pr=17848#step:9:224

Looks like a sporadic error where clearing an OrderedDict during iteration can sometimes raise with a message of OrderedDict mutated during iteration instead of the expected message of OrderedDict changed size during iteration.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[hauntsaninja]

Interesting that you got two failures in a row, I wonder if latest 3.13 branch changed something. (Test is easy to fix on our side, but could be worth checking whether CPython's behaviour is expected or not)

Thanks for spotting the issue and this fix, though!

[brianschubert]

Looks like this changed in python/cpython@c6c3d97 (python/cpython#121329). After that commit, the error message switches to OrderedDict mutated during iteration.