Add GitHub action to check for DAG import errors

[tahaislam]

What this pull request accomplishes:

Add GitHub action to check for:

• DAG import errors
• empty DAGs

Issue(s) this solves:

• Closes #796

What, in particular, needs to reviewed:

• the test script and the workflow yml file

What needs to be done by a sysadmin after this PR is merged

[radumas]

This looks good. I think we need to document this somewhere in a README, and what would need to be periodically updated (upstream GitHub actions? the Python version for the GH Action?)

What should someone do when they add a new DAG? add more mock variables to the test py?

[tahaislam]

This looks good. I think we need to document this somewhere in a README, and what would need to be periodically updated (upstream GitHub actions? the Python version for the GH Action?)

What should someone do when they add a new DAG? add more mock variables to the test py?

I think it's more suitable to add these instructions in this issue as part of the new section: "Adding new DAGs"