Do not crash in "too few arguments" error with anon argument (#3313)

Fix #3312

Author: sixolet

mypy/messages.py

@@ -575,7 +575,7 @@ def too_few_arguments(self, callee: CallableType, context: Context,
                 msg = 'Missing positional argument'
             else:
                 msg = 'Missing positional arguments'
-            if callee.name and diff:
+            if callee.name and diff and all(d is not None for d in diff):
                 msg += ' "{}" in call to {}'.format('", "'.join(diff), callee.name)
         else:
             msg = 'Too few arguments'

test-data/unit/check-functions.test

@@ -2039,3 +2039,8 @@ l: List[int] = make_list()
 
 bad = make_list()  # E: Need type annotation for variable
 [builtins fixtures/list.pyi]
+
+[case testAnonymousArgumentError]
+def foo(__b: int, x: int, y: int) -> int: pass
+foo(x=2, y=2)  # E: Missing positional argument
+foo(y=2)  # E: Missing positional arguments