[CT-1591] [Bug] New python files .py cause parsing and compilation errors

[hope-wat]

Is this a new bug in dbt-core?

• I believe this is a new bug in dbt-core
• I have searched the existing issues, and I could not find an existing issue for this bug

Current Behavior

When you open a new python file .py it causes a parsing and compilation error until the model code is inputted.

def model(dbt, session):
 ...
    return df

This is not great for mid development experience and blocks the ability to run any of your other models simply because you started working on a .py file.
https://www.loom.com/share/cea560725928459cb896a398289369e2

Expected Behavior

No error compilation for new and empty python files.
Allow you to run other models in your pipeline during python development.

Steps To Reproduce

1. In dbt cloud on v1.3
2. Create a new .py file without any code

Relevant log output

No response

Environment

- OS:
- Python:
- dbt:

Which database adapter are you using with dbt?

No response

Additional Context

No response

[dbeatty10]

Thanks for discovering and reporting this @hope-wat !

Per your initial convo with @jtcohen6 in internal Slack the fix would just be:

avoid parsing the file at all / raising that error if the file is totally empty

---

Side not

In your screenshot, it contains a phrase which isn't the most clear:

dbt only allow one model defined per python file

Thankfully an update to that piece was merged just yesterday in #6251. The new message will read:

dbt allows exactly one model defined per python file, found {x}

[jtcohen6]

This should be a really quick fix, and a nontrivial quality-of-life improvement! Let's try to make it happen.

[ChenyuLInx]

Talked about this live with @aranke
Acceptance criteria:
This is where the error being raised. We should make sure we add another unittest similar to this one but with empty file.