Use switch-case in PacketParserUtils.parseContentDepthWithoutRoundtrip()
Set conditionText to null if it's the empty string

This fixes e.g. IllegalArgumentException "'conflict' can not contain a
condition text", when receiving elements like

<stream:error>
  <conflict xmlns='urn:ietf:params:xml:ns:xmpp-streams'></conflict>
  <text xml:lang='' xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
    Replaced by new connection
  </text>
</sream:error>

(ejabberd does this)