KT-1934 & KT-10197:

Naive approximation for fake overriding signature:
use upper bound for flexible types.
Check delegation errors.

compiler/testData/diagnostics/tests/syntheticExtensions/javaProperties/KotlinOverridesJava.kt

@@ -24,7 +24,7 @@ fun foo(k: KotlinClass) {
     k.setSomething4("")
     k.something4 += ""
     k.setSomething4(<!NULL_FOR_NONNULL_TYPE!>null<!>)
-    k.something4 = null
+    k.something4 = <!NULL_FOR_NONNULL_TYPE!>null<!>
 
     useString(k.getSomething5())
     useString(k.something5)