fix(core): fix SlotController.isEmpty() #10472

Job	Run time
SSR Tests (Playwright)	1m 38s
Lint files	37s
Unit Tests (Web Test Runner)	1m 5s
Compile project (20)	1m 4s
Compile project (21)	1m 1s
Verify that build runs on Windows	2m 11s
Validate successful build on main	4s
Publish Playwright Report	16s
	7m 56s

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