avm2: Fix `Error.toString` regression

This was introduced by #14897, and to make sure this won't happen again I added a test.

Fixes #14901.

core/src/avm2/globals/Error.as

@@ -23,7 +23,7 @@ package {
 
 		prototype.toString = function():String {
 			var self:Error = this;
-			return self.message.length == 0 ? self.name : self.name + ": " + self.message;
+			return self.message !== "" ? self.name + ": " + self.message : self.name;
 		};
 	}
 }

tests/tests/swfs/avm2/issue_14901/Test.as

@@ -0,0 +1,8 @@
+package {
+import flash.display.Sprite;
+
+public class Test extends Sprite {}
+}
+
+var e = new Error(null);
+trace(e.toString());

tests/tests/swfs/avm2/issue_14901/output.txt

@@ -0,0 +1 @@
+Error: null

tests/tests/swfs/avm2/issue_14901/test.toml

@@ -0,0 +1 @@
+num_frames = 1