Fixes the undefined index error with the DAV property getlastmodified

[Maaxxs]

This PR fixes the undefined index {DAV:}getlastmodified error, which can occur in the DAV.php file.

The return value in the else clause is false if neither getetag nor getlastmodified is set. At least, I would argue that it should return false because false is returned if there is no response ($response === false), and in the exceptions, too.

It resolves some errors from issue #15363.

[tcitworld]

There was 1 failure:
1) OCA\Files_External\Tests\Storage\WebdavTest::testStat
Failed asserting that 0 is equal to 1647733789 or is greater than 1647733789. 
/drone/src/tests/lib/Files/Storage/Storage.php:318 

[Maaxxs]

Somehow I missed that typo yesterday night. Now it should get the proper modified time.

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/server/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22

[artonge]

Thanks for the fix @Maaxxs !

[solracsf]

Should this be backported?