Fix hasUpdated for webdav storage

[phil-davis]

Description

Unit tests get this fail on PHP 7.4

There was 1 failure:

1) Test\Files\Storage\DavTest::testHasUpdated with data set #4 (array('"oldetag"', 1), array('oldetag', 31), true)
Failed asserting that false matches expected true.

/drone/src/tests/lib/Files/Storage/DavTest.php:1306

Adjust the code so that it checks more carefully for non-existent array indexes etc.

The commit has been cherry-picked from PR #37280 where the demostration of PHP 7.4 tests is happening.

Note: there are no individual changelog entries being done for these PHP 7.4 code refactorings. An overall changelog entry for PHP 7.4 support will happen at the end.

Related Issue

How Has This Been Tested?

CI

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE