Fix PxLoaderTags#contains when comparing tag arrays with one element

The problem is:
    new PxLoaderTags('tag').contains(new PxLoaderTags('tag2')) ==> false
    new PxLoaderTags(['tag1']).contains(new PxLoaderTags(['tag2'])) ==> true

This happens because `contains` checks if the length of both arrays is one and
if this is the case it tries to compare the supposed singular value from both.
This then results in comparing null against null and then returning true, incorrectly.

PxLoader.js

@@ -349,7 +349,7 @@ function PxLoaderTags(values) {
     this.contains = function(other) {
         if (this.length === 0 || other.length === 0) {
             return false;
-        } else if (this.length === 1) {
+        } else if (this.length === 1 && this.value !== null) {
             if (other.length === 1) {
                 return this.value === other.value;
             } else {