HTML API: Fix phpdoc nullable types #7863

Job	Run time
Core running from build / Core running from build / Linux	2m 10s
Core running from src / Core running from src / Linux	2m 2s
Core running from build / Core running from build / Windows	4m 33s
Core running from src / Core running from src / Windows	3m 18s
Core running from build / Core running from build / MacOS	2m 7s
Core running from src / Core running from src / MacOS	1m 43s
Gutenberg running from build / Gutenberg running from build / Linux	7m 36s
Gutenberg running from src / Gutenberg running from src / Linux	7m 43s
Gutenberg running from build / Gutenberg running from build / Windows	11m 17s
Gutenberg running from src / Gutenberg running from src / Windows	10m 55s
Gutenberg running from build / Gutenberg running from build / MacOS	7m 29s
Gutenberg running from src / Gutenberg running from src / MacOS	7m 6s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 7m 59s

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