Prevent error when node.tags is None

[dbeatty10]

Resolves #10519

Problem

Dagster runs into an error when a manifest node has tags defined but it is None.

Solution

Skip iteration when tags is None or an empty list.

I didn't add any new tests here because I wasn't able to get an error when using the dbt CLI.

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.
• This PR doesn't have any new and modified functions for type annotations to apply to.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.88%. Comparing base (ff2726c) to head (e997616).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10520      +/-   ##
==========================================
- Coverage   88.89%   88.88%   -0.01%     
==========================================
  Files         180      180              
  Lines       22718    22718              
==========================================
- Hits        20196    20194       -2     
- Misses       2522     2524       +2     

Flag	Coverage Δ
integration	86.19% <100.00%> (-0.02%)	⬇️
unit	62.31% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Unit Tests	62.31% <100.00%> (ø)
Integration Tests	86.19% <100.00%> (-0.02%)	⬇️

[lamalex]

I forked dbt-core in my dagster system and this is almost exactly what I did. Lgtm!