Make sure a {DAV:}prop is not empty

[kesselb]

Port sabre-io/xml#158 to sabre/dav prop parser.

<?xml version="1.0" encoding="UTF-8"?>
<D:multistatus xmlns:D="DAV:">
  <D:response>
    <D:href>/alfresco/webdav/Webdav/</D:href>
    <D:propstat>
      <D:prop></D:prop>
      <D:status>HTTP/1.1 200 OK</D:status>
    </D:propstat>
    <D:propstat>
      <D:prop>
        <quota-available-bytes/></D:prop>
      <D:status>HTTP/1.1 404 Not Found</D:status>
    </D:propstat>
  </D:response>
</D:multistatus>

Is a multistatus propfind response by Alfresco. XMLReader or Sabre/XML is having troubles to parse the empty element.

Before	After
Faulty response (with no elements in propstat)	Correct response (with 1+ elements in propstat)

Report: nextcloud/server#16471

[codecov]

Codecov Report

Merging #1355 (43ea1d2) into master (7a638b6) will not change coverage.
The diff coverage is n/a.

❗ Current head 43ea1d2 differs from pull request most recent head c1da060. Consider uploading reports for the commit c1da060 to get more accurate results

@@            Coverage Diff            @@
##             master    #1355   +/-   ##
=========================================
  Coverage     97.31%   97.31%           
- Complexity     2806     2809    +3     
=========================================
  Files           174      174           
  Lines          7975     7975           
=========================================
  Hits           7761     7761           
  Misses          214      214           

Impacted Files	Coverage Δ
lib/DAV/Xml/Element/Response.php	100.00% <ø> (ø)

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[phil-davis]

@DeepDiver1975 @staabm I rebased this. I looks good to me. Please review.

[staabm]

looks good to me. but I cannot really judge.

[phil-davis]

Assigned myself so that I find this easily and followup. It would be nice to sort out the pending PRs in this repo and make a release in the next few days.