Fix regression in container check logic

This PR fixes the crash reported in python#8230,
by replacing the 'pass' with the 'continue', as suggested.

However, it does *not* fix the underlying root cause -- for example,
changing the lambda input type to `Optional[T]` bypasses the
check and triggers the crash again:

```
from typing import Callable, Optional, TypeVar

T = TypeVar('T')
def foo(f: Callable[[Optional[T]], bool], it: T) -> None: ...

foo(reveal_type(lambda x: x in [1, 2] and bool()), 3)
```

I'll update the issue with what I discovered while investigating this,
but I don't really have a deep understanding of how our
generics/function inference/deferred passes logic works, so didn't
feel comfortable volunteering a fix.

So, I settled just for fixing the regression.

mypy/checker.py

@@ -3915,7 +3915,7 @@ def find_isinstance_check_helper(self, node: Expression) -> Tuple[TypeMap, TypeM
 
                     # We only try and narrow away 'None' for now
                     if not is_optional(item_type):
-                        pass
+                        continue
 
                     collection_item_type = get_proper_type(builtin_item_type(collection_type))
                     if collection_item_type is None or is_optional(collection_item_type):

test-data/unit/check-optional.test

@@ -550,6 +550,17 @@ else:
     reveal_type(b)  # N: Revealed type is 'Union[__main__.A, None]'
 [builtins fixtures/ops.pyi]
 
+[case testInferInWithErasedTypes]
+from typing import TypeVar, Callable, Optional
+
+T = TypeVar('T')
+def foo(f: Callable[[T], bool], it: T) -> None: ...
+def bar(f: Callable[[Optional[T]], bool], it: T) -> None: ...
+
+foo(lambda x: x in [1, 2] and bool(), 3)
+bar(lambda x: x in [1, 2] and bool(), 3)
+[builtins fixtures/list.pyi]
+
 [case testWarnNoReturnWorksWithStrictOptional]
 # flags: --warn-no-return
 def f() -> None: