fix: ensure first argument on strpos is a string

[DeepDiver1975]

No description provided.

[codecov]

Codecov Report

Merging #1385 (fec8f14) into master (1f9bfa7) will decrease coverage by 0.18%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1385      +/-   ##
============================================
- Coverage     97.38%   97.19%   -0.19%     
  Complexity     2826     2826              
============================================
  Files           175      175              
  Lines          8021     8352     +331     
============================================
+ Hits           7811     8118     +307     
- Misses          210      234      +24     

Impacted Files	Coverage Δ
lib/DAV/Tree.php	100.00% <100.00%> (ø)
lib/DAV/Xml/Element/Response.php	96.00% <0.00%> (-4.00%)	⬇️
lib/CalDAV/ICSExportPlugin.php	97.22% <0.00%> (-1.36%)	⬇️
lib/DAVACL/Xml/Property/SupportedPrivilegeSet.php	95.23% <0.00%> (-1.32%)	⬇️
lib/DAVACL/Plugin.php	94.11% <0.00%> (-1.17%)	⬇️
lib/CalDAV/SharingPlugin.php	98.38% <0.00%> (-0.74%)	⬇️
lib/DAV/Server.php	96.62% <0.00%> (-0.58%)	⬇️
lib/CalDAV/Plugin.php	97.77% <0.00%> (-0.41%)	⬇️
lib/CardDAV/Plugin.php	96.70% <0.00%> (-0.22%)	⬇️
lib/DAV/Sync/Plugin.php	100.00% <0.00%> (ø)
... and 15 more

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[phil-davis]

owncloud/core#39702
PR owncloud/core#39703 has acceptance tests that demonstrate the problem.
Made the same edit as here, to sabre/dav/lib/DAV/Tree.php locally in my ownCloud dev installation, and reran the failing acceptance test and it passes.

[phil-davis]

LGTM