Fixes #144 - Zend_JsonTest::testDecodingInvalidJsonShouldRaiseAnException
Update Json.php

Replacing test on null with catch exception and rethrow the right Zend_Config_Exception - related to #144 (#150)
Update Json.php

Catch exception and rethrow instead of testing on null.
Update Json.php

Handling null value without json_last_error.
Update Json.php
Merge pull request #150 from siad007/patch-2

Fixes #144 - Unit test failed: Zend_JsonTest::testDecodingInvalidJsonShouldRaiseAnException