Get Namespace URI bound to a prefix in the current scope.

When requesting a Namespace URI by prefix, the following table describes the returned Namespace URI value for all possible prefix values:

`getNamespaceURI(prefix)` return value for specified prefixes
prefix parameter	Namespace URI return value
`DEFAULT_NS_PREFIX` ("")	default Namespace URI in the current scope or `XMLConstants.NULL_NS_URI("")` when there is no default Namespace URI in the current scope
bound prefix	Namespace URI bound to prefix in current scope
unbound prefix	`XMLConstants.NULL_NS_URI("")`
`XMLConstants.XML_NS_PREFIX` ("xml")	`XMLConstants.XML_NS_URI` ("http://www.w3.org/XML/1998/namespace")
`XMLConstants.XMLNS_ATTRIBUTE` ("xmlns")	`XMLConstants.XMLNS_ATTRIBUTE_NS_URI` ("http://www.w3.org/2000/xmlns/")
`null`	`IllegalArgumentException` is thrown

Parameters:

prefix prefix to look up

Returns: Namespace URI bound to prefix in the current scope

Exceptions:

IllegalArgumentException When prefix is null