[Tests] Migrate backend-independent tests from native to compiler/testData.

^KT-65979

compiler/testData/codegen/box/contracts/isNullString.kt

@@ -0,0 +1,12 @@
+// WITH_STDLIB
+import kotlin.test.*
+
+fun box(): String {
+    assertEquals("STR", nullableString("str"))
+
+    assertEquals("", nullableString(null))
+
+    return "OK"
+}
+
+private fun nullableString(string: String?): String = if (string.isNullOrBlank()) "" else "STR"