fix Trying to access array offset on value of type null in 7.4

[remicollet]

Fix for

4) Sabre\DAVACL\Xml\Property\PrincipalTest::testSimple
Trying to access array offset on value of type null

/work/GIT/sabre-dav/lib/DAV/Xml/Property/Href.php:62
/work/GIT/sabre-dav/tests/Sabre/DAVACL/Xml/Property/PrincipalTest.php:17

After this fix, still some other error in used library

There were 3 errors:

1) Sabre\HTTP\Auth\DigestTest::testInvalidDigest2
Trying to access array offset on value of type bool

/work/GIT/sabre-dav/vendor/sabre/http/lib/Auth/Digest.php:130
/work/GIT/sabre-dav/vendor/sabre/http/lib/Auth/Digest.php:98
/work/GIT/sabre-dav/vendor/sabre/http/tests/HTTP/Auth/DigestTest.php:101

2) Sabre\VObject\Parser\MimeDirTest::testParseError
fgets(): read of 8192 bytes failed with errno=9 Bad file descriptor

/work/GIT/sabre-dav/vendor/sabre/vobject/lib/Parser/MimeDir.php:278
/work/GIT/sabre-dav/vendor/sabre/vobject/lib/Parser/MimeDir.php:144
/work/GIT/sabre-dav/vendor/sabre/vobject/lib/Parser/MimeDir.php:90
/work/GIT/sabre-dav/vendor/sabre/vobject/tests/VObject/Parser/MimeDirTest.php:19

3) Sabre\DAV\Auth\Backend\AbstractDigestTest::testCheckNoHeaders
Trying to access array offset on value of type bool

/work/GIT/sabre-dav/vendor/sabre/http/lib/Auth/Digest.php:121
/work/GIT/sabre-dav/lib/DAV/Auth/Backend/AbstractDigest.php:106
/work/GIT/sabre-dav/tests/Sabre/DAV/Auth/Backend/AbstractDigestTest.php:18

[codecov]

Codecov Report

Merging #1187 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1187   +/-   ##
=========================================
  Coverage     97.17%   97.17%           
  Complexity     2872     2872           
=========================================
  Files           174      174           
  Lines          8067     8067           
=========================================
  Hits           7839     7839           
  Misses          228      228

Impacted Files	Coverage Δ	Complexity Δ
lib/DAV/Xml/Property/Href.php	100% <100%> (ø)	12 <0> (ø)	⬇️

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[evert]

lgtm. Maybe this function can also get a typehint or just a docblock update

[staabm]

thx remi, merging..