Make sure the arrays are arrays

[nickvergessen]

I don't know why, but on the call to preDelete the dav HookManager seems to not be registered yet, and then postDelete failed with foreach and null:
https://travis-ci.org/nextcloud/activity/jobs/328063705#L726

[codecov]

Codecov Report

Merging #7812 into master will decrease coverage by 1.64%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #7812 +/- ##
============================================
- Coverage 52.87% 51.23% -1.65% - Complexity 23658 24970 +1312 
============================================
Files 1449 1607 +158 Lines 80454 95008 +14554 Branches 0 1376 +1376 ============================================
+ Hits 42541 48674 +6133 - Misses 37913 46334 +8421

Impacted Files	Coverage Δ	Complexity Δ
apps/dav/lib/HookManager.php	51.78% <ø> (ø)	15 <0> (ø)	⬇️
.../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 348 more

[rullzer]

Still we should investigate the weird bug...

[MorrisJobke]

@nickvergessen Could you open the backport PRs?

[nickvergessen]

#7943