substring filter fixes for clients

lib/filters/index.js

@@ -154,9 +154,9 @@ function _parseString(str) {
           } else {
             filters.push(new SubstringFilter({
               attribute: stack[i].attribute,
-              initial: vals.shift(),
-              'final': vals.pop(),
-              any: vals
+              initial: vals.shift() || null,
+              'final': vals.pop() || null,
+              any: vals || null
             }));
           }
         }

lib/filters/substr_filter.js

@@ -16,9 +16,9 @@ function SubstringFilter(options) {
     if (!options.attribute || typeof(options.attribute) !== 'string')
       throw new TypeError('options.attribute (string) required');
     this.attribute = options.attribute;
-    this.initial = options.initial;
+    this.initial = options.initial || null;
     this.any = options.any ? options.any.slice(0) : [];
-    this['final'] = options['final'];
+    this['final'] = options['final'] || null;
   } else {
     options = {};
   }
@@ -45,13 +45,19 @@ module.exports = SubstringFilter;
 
 SubstringFilter.prototype.toString = function() {
   var str = '(' + this.attribute + '=';
+
   if (this.initial)
-    str += this.initial + '*';
+    str += this.initial;
+
+  str += '*';
+
   this.any.forEach(function(s) {
     str += s + '*';
   });
+
   if (this['final'])
     str += this['final'];
+
   str += ')';
 
   return str;