[FixBug] Fixed bug when attribute is actually an "0"

When we set an attribute in XML file like this: "some_value"="0"
(The value with this attribute accept any integer include "0")
Then, we use XmlHelper to get the value with: "$value = XMLHelper::get($element, 'some_value', 3);"
The value will be "3" instead of "0"

These changes could fix this kind of problem