oEmbed: Fix PHP warnings when a registered provider is malformed #29343

Job	Run time
Core running from build / Linux	2m 31s
Core running from build / Windows	3m 40s
Core running from build / MacOS	4m 35s
Core running from src / Windows	3m 49s
Core running from src / MacOS	2m 0s
Core running from src / Linux	2m 19s
Slack Notifications	0s
Failed workflow tasks	0s
	18m 54s

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