fix: give the remaining app config settings an empty-string default

[oc-tmueller]

Fixes #644.

The bug

AppConfig::getAppValue() only passes a default down to IConfig for keys listed in
AppConfig::$defaults. Five keys were missing from that list, so they returned null until an
admin saved the setting: edit_groups, wopi_url, test_wopi_url, doc_format and
menu_option.

On PHP 8.1 and later that has three visible consequences:

null reaches	effect
explode() in DocumentController::isAllowedEditor()	an ERROR in owncloud.log on every single document open — the reported bug
DiscoveryService::getWopiUrl(): string	TypeError, because a string-typed method may not return null
p() in templates/settings-admin.php (3 places)	deprecation when the admin settings page is opened

The fix

The five missing keys get an '' default, which is how every other string setting in the class is
already handled. No other production code changes.

No consumer branches on these values being null, so the change is behaviour preserving — with one
exception it incidentally fixes: the admin template hides the test server block when
test_wopi_url === '', which could never be true while the value was null. With a test group set
but no test server URL the block used to render even though js/settings-admin.js left its
checkbox unchecked; it is now hidden, which is what that condition always meant to express.

zoteroAPIPrivateKey is deliberately left out: it is a user value and both of its readers guard
with !empty().

Verification

Run against a real ownCloud 11 install (owncloudci/php:8.3, sqlite), reading the values through
the real IConfig rather than a mock, with and without the fix:

=== WITHOUT the fix ===            === WITH the fix ===
edit_groups      => NULL           edit_groups      => ''
wopi_url         => NULL           wopi_url         => ''
test_wopi_url    => NULL           test_wopi_url    => ''
doc_format       => NULL           doc_format       => ''
menu_option      => NULL           menu_option      => ''
RAISED: explode(): Passing null to parameter #2 ($string) of type string is deprecated
getWopiUrl() THREW TypeError:      getWopiUrl() => ''
  Return value must be of type
  string, null returned

• tests/unit/AppConfigTest.php gains a data-provider case per key. Its with(..., identicalTo(''))
  constraint is the load-bearing part: all five cases fail with
  Failed asserting that null is identical to '' when the $defaults entries are removed.
  (A loose with(..., '') does not catch this — PHPUnit compares loosely and null == ''.)
• Full unit suite green: 83 tests, 209 assertions.
• php-cs-fixer clean (0 of 45 files), phpstan level 5 "No errors", phan clean.

Branch scope

master only. The missing defaults exist on 4.2 too, but PHP 7.4 does not surface the
deprecation there, and the 4.2.4 artifact is Collabora's to build (#643) — a commit on 4.2 would
change the tree they were pointed at.

🤖 Generated with Claude Code

[phil-davis]

There are a few other releases 4.2.1 4.2.2 4.3.2 that have gone missing from this section.
Maybe fix that in a follow-up PR (not directly related to this)

[oc-tmueller]

Agreed, and done as a follow-up: #646.

It turned out to be a bit more than missing entries. Five headings had no definition at all —
4.2.1, 4.2.2, 4.2.3, 4.3.0, 4.3.1 (there is no 4.3.2 yet) — and on top of that two of the
existing definitions pointed at tags that were never created, so they were live 404s:

• [4.0.0] → compare/v3.0.1...v4.0.0 — v3.0.1 was only ever tagged -beta.1 / -rc.1
• [3.0.1] → same missing tag, and an orphan: there is no ## [3.0.1] section
• [2.4.0] → compare/v2.2.0...v2.4.0 — v2.4.0 is the oldest tag in the repo

#646 fetches every link in the block afterwards: all 15 return 200, and every section heading has
exactly one definition.

One thing worth knowing: #646 touches this same contiguous block, so the two conflict by
construction. Merging this PR first and letting me rebase #646 is the easy order.