Avoid dev session region errors when no region is specified

[thedadams]

When starting a dev session on a new app without setting a region on the app, an error might crop up complaining about regions. This does not happen if a region is explicitly set on the app.

Therefore, since users don't explicitly create dev session objects, setting the region for them when the dev session region is unset is reasonable.

Checklist

• The title of this PR would make a good line in Acorn's Release Note's Changelog
• The title of this PR ends with a link to the main issue being address in parentheses, like: This is a title (#1216). Here's an example
• All relevant issues are referenced in the PR description. NOTE: don't use GitHub keywords that auto-close issues
• Commits follow contributing guidance
• Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
• Changes to user-facing functionality, API, CLI, and upgrade impacts are clearly called out in PR description
• PR has at least two approvals before merging (or a reasonable exception, like it's just a docs change)

[cjellick]

For https://github.com/acorn-io/manager/issues/1505