handleFlush: do not use inherent nullability for unknown values

Fixes IDEA-172844 False positive "Method invocation 'methodName' may produce NPE" in while loop condition

java/java-tests/testData/inspection/dataFlow/fixture/NullableGetterInLoop.java

@@ -0,0 +1,27 @@
+import org.jetbrains.annotations.*;
+
+// IDEA-172844
+abstract class SimpleClass {
+
+  @Nullable
+  protected MyType findSelfOrInnerByNonQualifiedName() {
+    MyType nearestOuter = getOwnerParentOfType(MyType.class);
+    if (nearestOuter != null) {
+      MyType currentOuter = nearestOuter;
+      while (currentOuter.getOuterClass() != null) { // False positive error here
+        currentOuter = currentOuter.getOuterClass();
+      }
+    }
+
+    return null;
+  }
+
+  @Nullable
+  abstract MyType getOwnerParentOfType(Class<MyType> apexPsiTypeDeclarationClass);
+
+  static abstract class MyType {
+
+    @Nullable
+    abstract MyType getOuterClass();
+  }
+}