EAGLESIX-727 Expressions... DateToString. evaluateInternal() change to return a null for null input

lib/pipeline/expressions/DateToStringExpression.js

@@ -192,8 +192,9 @@ proto.getOpName = function getOpName() {
 proto.evaluateInternal = function evaluateInternal(vars) {
     var date = this.operands[0].evaluateInternal(vars);
 
-    if (!date || date == null)
-    	return formatDate(this._format, date);
+    if (date === undefined || date === null)
+    	return null;
+	return formatDate(this._format, date);
 };
 
 proto.addDependencies = function addDependencies(depsTracker) {