feat: Support CLI flag to discard env vars with empty value

[korya]

Instead of failing.

This eliminates the need to wrap lk invocations with custom logic for manually filtering out such values. For example, this is a real script for deploying using lk agent deploy:

rm -f .env.lk
sed '/^#/d' .env | grep -vE '^[A-Za-z_][A-Za-z0-9_]*=$' > .env.lk
lk agent deploy --secrets-file .env.lk

With the flag, this would be just:

lk agent deploy --secrets-file .env --ignore-empty

Related:

• https://github.com/livekit/livekit-cli/issues/678
• https://github.com/livekit/livekit-cli/pull/724

[CLAassistant]

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