List of steps to reproduce (step by step, including full links if applicable):

1. Enabled debug log for [squid] at least
2. Open any page to edit ?action=edit
3. Save without changing any content (or add any newline at the end, as long as the edit is considered null edit)

What happens?:

1. Observed that CdnCacheUpdate::purge is not called

What should have happened instead?:
There should be CdnCacheUpdate::purge calls, like a regular edits

[MessageCache] MessageCache::load: Loading en... local cache is empty, global cache is expired/volatile, loading from database
[squid] CdnCacheUpdate::purge: xxxxx

For reference:

1. the purge call can be observed when using MediaWiki 1.33.3
2. calling action=purge on the page separately correctly triggers CDN purge

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.:
MediaWiki 1.37.2