Steps to replicate the issue (include links if applicable):

• install mediawiki 1.39.7 on Ubuntu 20.04
• truncate tables per the documentation
• run grabFiles.php with appropriate parameters

What happens?:
[1196950134fc482fa81cbe7a] [no req] TypeError: Typed property GrabFiles::$endDate must be string, null used

What should have happened instead?:
script should have started to download images from wiki

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia): 1.39.7 on ubuntu 20.04

Other information (browser name/version, screenshots, etc.):
Change-Id I6577520aae5debe36a9c2aff000a37402b7e0dbe is what broke the script.
Using code prior to this commit id has no issue, this needs to be reversed