[Backport maintenance/2.17.x] Fix a crash when __all__ exists but cannot be inferred

doc/whatsnew/fragments/8740.bugfix

@@ -0,0 +1,3 @@
+Fix a crash when ``__all__`` exists but cannot be inferred.
+
+Closes #8740

pylint/checkers/variables.py

@@ -2982,7 +2982,10 @@ def _check_module_attrs(
     def _check_all(
         self, node: nodes.Module, not_consumed: dict[str, list[nodes.NodeNG]]
     ) -> None:
-        assigned = next(node.igetattr("__all__"))
+        try:
+            assigned = next(node.igetattr("__all__"))
+        except astroid.InferenceError:
+            return
         if isinstance(assigned, util.UninferableBase):
             return
         if assigned.pytype() not in {"builtins.list", "builtins.tuple"}:

tests/functional/u/undefined/undefined_all_variable_edge_case.py

@@ -0,0 +1,5 @@
+"""Edge case: __all__ exists in module's locals, but cannot be inferred.
+
+Other tests for undefined-all-variable in tests/functional/n/names_in__all__.py"""
+
+__all__ += []  # [undefined-variable]

tests/functional/u/undefined/undefined_all_variable_edge_case.txt

@@ -0,0 +1 @@
+undefined-variable:5:0:5:7::Undefined variable '__all__':UNDEFINED