fix(core): fix SlotController.isEmpty() #10471

Job	Run time
Compile project (21)	1m 6s
Verify that build runs on Windows	2m 3s
Compile project (20)	1m 7s
Lint files	41s
SSR Tests (Playwright)	1m 33s
Unit Tests (Web Test Runner)	1m 1s
Validate successful build on main	4s
Publish Playwright Report	21s
	7m 56s

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