Feeds: Avoid fatal error with empty `blog_charset` value. #18878

Job	Run time
QUnit Tests / Run QUnit tests	2m 4s
Slack Notifications / Prepare notifications	3s
Slack Notifications / Success notifications	7s
Slack Notifications / Cancelled notifications	0s
Slack Notifications / Fixed notifications	0s
Slack Notifications / Failure notifications	0s
Failed workflow tasks	0s
	2m 14s

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