[CB-7418][DirectoryEntry] Added fullPath variable as part of condition

As well as NativeURL property, the fullPath variable should be part of
the condition to avoid that a null value becomes a path, by adding a
slash at the end of the undefined string.
So in order to add a slash to it, fullPath should contain a value.

Author: martincgg

www/DirectoryEntry.js

@@ -38,7 +38,7 @@ var argscheck = require('cordova/argscheck'),
 var DirectoryEntry = function(name, fullPath, fileSystem, nativeURL) {
 
     // add trailing slash if it is missing
-    if (!/\/$/.test(fullPath)) {
+    if ((fullPath) && !/\/$/.test(fullPath)) {
         fullPath += "/";
     }
     // add trailing slash if it is missing