Complex augmented assignment test.

Fix string literal creation for empty string template.

compiler/testData/ir/irText/arrayAugmentedAssignment2.kt

@@ -0,0 +1,11 @@
+interface IA {
+    operator fun get(index: String): Int
+}
+
+interface IB {
+    operator fun IA.set(index: String, value: Int)
+}
+
+fun IB.test(a: IA) {
+    a[""] += 42
+}