"Remove redundant toString": fix false positive for nested nullable

So #KT-24557 Fixed

idea/testData/intentions/removeRedundantCallsOfConversionMethods/nullable2.kt

@@ -0,0 +1,7 @@
+// IS_APPLICABLE: false
+// WITH_RUNTIME
+data class Foo(val name: String)
+
+fun nullable2(foo: Foo?) {
+    val s: String = foo?.name.toString()<caret>
+}