[Wasm] Fix generic call return type for functions with default parameters

compiler/testData/codegen/box/casts/genericReturnCast.kt

@@ -0,0 +1,17 @@
+fun <T : Any?> someFunc(a: T, arg: T? = null): T = arg ?: a
+
+fun box(): String {
+    var key: String? = null
+    key = someFunc("a")
+    if (key != "a") return "FAIL1"
+    key = someFunc("a", "b")
+    if (key != "b") return "FAIL2"
+
+    var key1: Int? = null
+    key1 = someFunc(42)
+    if (key1 != 42) return "FAIL3"
+    key1 = someFunc(42, 24)
+    if (key1 != 24) return "FAIL3"
+
+    return "OK"
+}