Override/Implement: Make return type non-nullable (platform collection case) when overriding Java method

#KT-13455 Fixed

idea/testData/codeInsight/overrideImplement/platformCollectionTypes/foo/A.java

@@ -0,0 +1,7 @@
+package foo;
+
+import java.util.List;
+
+public interface A {
+    List<String> foo();
+}

idea/testData/codeInsight/overrideImplement/platformCollectionTypes/foo/Impl.kt

@@ -0,0 +1,5 @@
+import foo.A
+
+class B : A {
+    <caret>
+}

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

@@ -0,0 +1,7 @@
+import foo.A
+
+class B : A {
+    override fun foo(): MutableList<String> {
+        <selection><caret>TODO("not implemented") //To change body of created functions use File | Settings | File Templates.</selection>
+    }
+}