fix(core): fix SlotController.isEmpty() (#2906) #10495

Job	Run time
SSR Tests (Playwright)	1m 30s
Verify that build runs on Windows	2m 10s
Compile project (20)	1m 12s
Compile project (21)	1m 6s
Unit Tests (Web Test Runner)	1m 5s
Lint files	33s
Validate successful build on main	3s
Publish Playwright Report	17s
	7m 56s

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