Convert to block body: specify non-nullable type when body expression is platform type and overriden method is non-nullable type

#KT-12222 Fixed

idea/testData/intentions/convertToBlockBody/overrideWithPlatformType.kt

@@ -0,0 +1,8 @@
+// WITH_RUNTIME
+interface Foo {
+    fun foo(): String
+}
+
+class Bar : Foo {
+    override fun <caret>foo() = java.lang.String.valueOf(42)
+}

idea/testData/intentions/convertToBlockBody/overrideWithPlatformType.kt.after

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+interface Foo {
+    fun foo(): String
+}
+
+class Bar : Foo {
+    override fun foo(): String {
+        return java.lang.String.valueOf(42)
+    }
+}