Undefined variables in let widget: Revert fix for #7270

Reverts 8c378e0d24 and adds a test

core/modules/widgets/let.js

@@ -74,9 +74,7 @@ LetWidget.prototype.getVariableInfo = function(name,options) {
 			text: this.currentValueFor[name]
 		};
 	}
-	return Widget.prototype.getVariableInfo.call(this,name,$tw.utils.extend(Object.create(null),options,{
-		defaultValue: ""
-	}));
+	return Widget.prototype.getVariableInfo.call(this,name,options);
 };
 
 /*

editions/test/tiddlers/tests/data/widgets/LetWidgetSelfReferences.tid

@@ -0,0 +1,15 @@
+title: Widgets/LetWidgetSelfReferences
+description: Test let widget self references
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\whitespace trim
+<$let default={{{ [[default]is[variable]then[aa]else[bb]] }}} >
+<<default>>
+</$let>
++
+title: ExpectedResult
+
+<p>bb</p>