IDEA-73344: Writing unknown value to a nullable variable must reset it's nullability status to unknown, rather than nullable.

java/java-tests/testData/inspection/dataFlow/NullableLocalVariable/src/Npe.java

@@ -7,7 +7,7 @@ public class Npe {
      o.hashCode(); // NPE
    }
 
-   Object foo() {
+   @Nullable Object foo() {
      return null;
    }
-}
+}