Override/Implement: Prefer not-nullable return type when overriding Java method without nullability annotation

#KT-12381 Fixed
(cherry picked from commit 2b87f8e)

idea/testData/codeInsight/overrideImplement/javaInterfaceMethod/foo/Impl.kt.after

@@ -1,7 +1,7 @@
 import foo.Intf
 
 class Impl(): Intf {
-    override fun getFooBar(): String? {
+    override fun getFooBar(): String {
         <selection><caret>throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.</selection>
     }
 }