modules/presence_xml: fixed issue with parsing some pres-rules documents

- pres-rules that contain external links in a <conditions /> node don't
  have to have <identity /> nodes in the <conditions /> node.
- Kamailio returns an error when it finds a <conditions /> node with
  no <identity /> node inside it.
- Kamailio doesn't support external links, but it should skip over
  the <conditions /> nodes that contain them rather than return
  an error as later <conditions /> nodes may contain entries it can
  work with.
(cherry picked from commit ac99219b0a850e0305eaf661b028536cfc28e2cb)