Fix package version validation to handle missing property

[benlevycmp]

Resolves #12649

Problem

Previously, the custom error would not be raised if the version property was missing entirely, rather than simply falsy, and only a cryptic KeyError would be raised.

Solution

Replaces direct key access with the safer get() method.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[dbeatty10]

Thanks for opening this @benlevycmp !

Please do the following so we can review:

1. Add a changelog entry
2. Add a unit test or functional test where version is omitted for a hub package (examples here, here, here, etc).