fix: prevent possible error on embedded component disconnect

[web-padawan]

Description

This fixes the following error which sometimes occurs when switching between docs pages with Java examples:

Uncaught TypeError: Cannot read properties of undefined (reading 'disconnected')
    at ContextMenuPresentationWc.disconnectedCallback (:8000/vaadin/VAADIN/generated/flow/web-components/context-menu-presentation-wc.js:131:18)
    at removeChild (chunk-UXDD7MME.js:8456:26)

I'm not sure how to test this but it makes sense to check for object being defined before calling its methods anyway.

Type of change

• Bugfix

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

Test Results

1 142 files  ± 0  1 142 suites  ±0   1h 25m 21s ⏱️ + 1m 29s
7 464 tests ± 0  7 414 ✅ ± 0  50 💤 ±0  0 ❌ ±0 
7 797 runs  +17  7 737 ✅ +17  60 💤 ±0  0 ❌ ±0 

Results for commit 7405061. ± Comparison against base commit 1937249.