Fix crash when indexing TypedDict with empty key (#15392)

mypy/messages.py

@@ -2989,8 +2989,9 @@ def _real_quick_ratio(a: str, b: str) -> float:
 
 
 def best_matches(current: str, options: Collection[str], n: int) -> list[str]:
+    if not current:
+        return []
     # narrow down options cheaply
-    assert current
     options = [o for o in options if _real_quick_ratio(current, o) > 0.75]
     if len(options) >= 50:
         options = [o for o in options if abs(len(o) - len(current)) <= 1]

test-data/unit/check-typeddict.test

@@ -2873,3 +2873,15 @@ foo({"foo": {"e": "foo"}})  # E: Type of TypedDict is ambiguous, none of ("A", "
                             # E: Argument 1 to "foo" has incompatible type "Dict[str, Dict[str, str]]"; expected "Union[A, B]"
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-typeddict.pyi]
+
+[case testTypedDictMissingEmptyKey]
+from typing_extensions import TypedDict
+
+class A(TypedDict):
+    my_attr_1: str
+    my_attr_2: int
+
+d: A
+d['']  # E: TypedDict "A" has no key ""
+[builtins fixtures/dict.pyi]
+[typing fixtures/typing-typeddict.pyi]