[K2, MPP] Fix actualization of nullability-based overloads

^KT-58043 Fixed

compiler/testData/codegen/box/multiplatform/k2/basic/expectActualNullabilityBasedOverloads.kt

@@ -0,0 +1,20 @@
+// IGNORE_BACKEND: JVM, JVM_IR
+// IGNORE_BACKEND_K1: JS, JS_IR, JS_IR_ES6, NATIVE
+// !LANGUAGE: +MultiPlatformProjects
+
+// MODULE: common
+// FILE: common.kt
+
+expect fun o(x: String?): String?
+expect fun k(x: String?): String?
+
+fun box() = o(null) + k(null)
+
+// MODULE: platform()()(common)
+// FILE: platform.kt
+
+fun o(x: String) = ""
+actual fun o(x: String?): String? = "O"
+
+actual fun k(x: String?): String? = "K"
+fun k(x: String) = ""