Add test for toString of nullable abstract

tests/unit/src/unit/issues/Issue12019.hx

@@ -0,0 +1,48 @@
+package unit.issues;
+
+private abstract MyStringA(Null<String>) from Null<String> {
+	function toString() {
+		if (this == null)
+			return "EMPTY";
+		return this;
+	}
+}
+
+private typedef NullableString = Null<String>;
+
+private abstract MyStringB(NullableString) from NullableString {
+	function toString() {
+		if (this == null)
+			return "EMPTY";
+		return this;
+	}
+}
+
+class Issue12019 extends unit.Test {
+	final a:MyStringA = null;
+	final b:MyStringB = null;
+
+	var oldTrace:(Dynamic, ?Null<haxe.PosInfos>) -> Void;
+
+	function setup() {
+		oldTrace = haxe.Log.trace;
+	}
+
+	function teardown() {
+		haxe.Log.trace = oldTrace;
+	}
+
+	function testTrace() {
+		haxe.Log.trace = function(v, ?infos) {
+			eq("EMPTY", v);
+		};
+
+		trace(a);
+		trace(b);
+	}
+
+	function testConcatenate() {
+		eq("Concatenated: EMPTY", "Concatenated: " + a);
+		eq("Concatenated: EMPTY", "Concatenated: " + b);
+	}
+}