Fix strpos if needle is not found

[MorrisJobke]

Found while testing strict_typing for PHP 7+ (#7392). If $script does not contain a / then it should extract a string of the length 0.

[codecov]

Codecov Report

Merging #7816 into master will decrease coverage by 1.64%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #7816      +/-   ##
============================================
- Coverage     52.87%   51.23%   -1.65%     
- Complexity    23658    24971    +1313     
============================================
  Files          1449     1607     +158     
  Lines         80454    95008   +14554     
  Branches          0     1376    +1376     
============================================
+ Hits          42541    48673    +6132     
- Misses        37913    46335    +8422

Impacted Files	Coverage Δ	Complexity Δ
lib/private/legacy/app.php	54.06% <0%> (ø)	222 <0> (+1)	⬆️
.../tests/Unit/Collaboration/CommentersSorterTest.php	25.55% <0%> (-66.45%)	6% <0%> (ø)
apps/sharebymail/tests/SettingsTest.php	52.17% <0%> (-47.83%)	3% <0%> (ø)
lib/private/Security/RateLimiting/Limiter.php	55.55% <0%> (-44.45%)	5% <0%> (ø)
settings/Controller/EncryptionController.php	54.71% <0%> (-38.84%)	8% <0%> (ø)
settings/Controller/GroupsController.php	64.61% <0%> (-35.39%)	9% <0%> (ø)
...ps/comments/tests/Unit/AppInfo/ApplicationTest.php	69.56% <0%> (-30.44%)	4% <0%> (ø)
lib/private/AvatarManager.php	71.42% <0%> (-28.58%)	4% <0%> (ø)
apps/user_ldap/lib/Configuration.php	42.02% <0%> (-27.86%)	87% <0%> (ø)
apps/encryption/lib/Command/EnableMasterKey.php	75% <0%> (-25%)	5% <0%> (ø)
... and 347 more