ci: fix poetry lock workflow for missing/blank file

[devin-ai-integration]

Copy poetry lock workflow fix from airbyte-python-cdk to check for missing/blank lock file.

This PR copies the changes from airbyte-python-cdk#284 to ensure that when we run the /poetry-lock slash command, the job first checks for a blank or missing lock file. If blank or missing, it gets the lock file from the main branch before running poetry lock.

Link to Devin run: https://app.devin.ai/sessions/f2f03d0c39cb4355924e0243ddbae7e2

[devin-ai-integration]

🤖 Devin AI Engineer

Original prompt from Aaron:

@Devin - copy the changes from this PR into the PyAirbyte repo. <https://github.com/airbytehq/airbyte-python-cdk/pull/284|https://github.com/airbytehq/airbyte-python-cdk/pull/284>.

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