fix(core): fix SlotController.isEmpty() #10492

Job	Run time
Lint files	34s
Unit Tests (Web Test Runner)	1m 7s
SSR Tests (Playwright)	1m 36s
Compile project (21)	1m 7s
Compile project (20)	1m 2s
Verify that build runs on Windows	1m 58s
Validate successful build on main	4s
Publish Playwright Report	22s
	7m 50s

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