ci: fix poetry lock workflow for missing/blank file

[devin-ai-integration]

Fetch lock file from main if missing or blank before re-running poetry lock.

Link to Devin run: https://app.devin.ai/sessions/1838ffc1ce88495cb7a3dac02c041c71

[devin-ai-integration]

🤖 Devin AI Engineer

Original prompt from Aaron:

@Devin - Please modify the CI workflow in the CDK so that when we run the `/poetry-lock` slash command, the job first checks for a blank or missing lock file. If blank or missing, get the lock file from the main branch before running `poetry lock` - rather than running with no baseline lock file.

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