[xmlparser-stax] Fix potential IAE in StaxXmlPullParser.getNamespace()
StAX's NamespaceContext.getNamespaceURI(String) is allowed to throw an
IllegalArgumentException in case the argument is null. We simply
re-use getNamespace(String) which will substitute the default
namespace if the argument is null to avoid this.
Connection closed due to an exception
java.lang.IllegalArgumentException: Illegal to pass null as argument
at com.ctc.wstx.sr.NsInputElementStack.getNamespaceURI(NsInputElementStack.java:486)
at org.jivesoftware.smack.xml.stax.StaxXmlPullParser.getNamespace(StaxXmlPullParser.java:113)
at org.jivesoftware.smack.AbstractXMPPConnection.parseFeatures(AbstractXMPPConnection.java:1852)
at org.jivesoftware.smack.AbstractXMPPConnection.parseFeaturesAndNotify(AbstractXMPPConnection.java:1887)
at org.jivesoftware.smack.tcp.XMPPTCPConnection.access$1300(XMPPTCPConnection.java:130)
at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketReader.parsePackets(XMPPTCPConnection.java:983)
at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketReader.access$700(XMPPTCPConnection.java:913)
at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketReader$1.run(XMPPTCPConnection.java:936)
at java.lang.Thread.run(Thread.java:748)
