Return null for the name space prefix while it is the default name space

Per the Java doc of the XMLStreamReader, it requires null value is returned while it is default name space. I knew that there are some discussion in the past for it, and the spec seems not clarify this clearly in each scenario. For the reason to keep the compatibility of jsr 107, it would be better to have a flag to change the default behavior.

getNamespacePrefix
String getNamespacePrefix(int index)
Returns the prefix for the namespace declared at the index. Returns null if this is the default namespace declaration

Parameters:
index - the position of the namespace declaration
Returns:
returns the namespace prefix
Throws:
IllegalStateException - if this is not a START_ELEMENT, END_ELEMENT or NAMESPACE