docs: clarify nullish dot-notation components

[nightt5879]

Fixes #17493

Summary

• Clarify that dot-notation component tags still resolve the dotted expression at render time.
• Show guarding a nullable component object before rendering a dotted component tag.

Verification

• npx --yes prettier@3.2.4 --check documentation/docs/03-template-syntax/01-basic-markup.md
• git diff --check

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Latest commit: cfef729

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