Fix None assignments with fine-grained cache (#11574)

Problem: When using the fine-grained cache, we raise an assertion in
some situations when an attribute is defined as `None`.

Solution: Remove truthiness check to ensure that `None` is an allowed
type.

Fixes: #8682
Fixes: #11456

Author: christianbundy

mypy/server/deps.py

@@ -481,8 +481,11 @@ def get_non_partial_lvalue_type(self, lvalue: RefExpr) -> Type:
             return UninhabitedType()
         lvalue_type = get_proper_type(self.type_map[lvalue])
         if isinstance(lvalue_type, PartialType):
-            if isinstance(lvalue.node, Var) and lvalue.node.type:
-                lvalue_type = get_proper_type(lvalue.node.type)
+            if isinstance(lvalue.node, Var):
+                if lvalue.node.type:
+                    lvalue_type = get_proper_type(lvalue.node.type)
+                else:
+                    lvalue_type = UninhabitedType()
             else:
                 # Probably a secondary, non-definition assignment that doesn't
                 # result in a non-partial type. We won't be able to infer any

test-data/unit/fine-grained.test

@@ -9733,3 +9733,28 @@ class C:
 [out]
 ==
 main:5: error: Unsupported left operand type for + ("str")
+[case testNoneAttribute]
+from typing import Generic, TypeVar
+
+T = TypeVar('T', int, str)
+
+class ExampleClass(Generic[T]):
+    def __init__(
+        self
+    ) -> None:
+        self.example_attribute = None
+[out]
+==
+[case testStrictNoneAttribute]
+# flags: --strict-optional
+from typing import Generic, TypeVar
+
+T = TypeVar('T', int, str)
+
+class ExampleClass(Generic[T]):
+    def __init__(
+        self
+    ) -> None:
+        self.example_attribute = None
+[out]
+==