[Bug] Microbatch models with only one batch raise unhandled error

[QMalcolm]

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

Currently on 1.9.latest if you run a microbatch model with only one batch (can be achieved by setting lookback=0 for the model), an unhandled exception gets raised (but doesn't actually break anything?)

Expected Behavior

There shouldn't be an unhandled exception for simply having a microbatch model with lookback=0

Steps To Reproduce

1. Ensure you're running on dbt-core 1.9.latest via pip install -e <path>/<to>/<core>/<repo>/dbt-core/core
   • repository must be checked out on 1.9.latest branch
2. Create a dbt project with a microbatch incremental model (with lookback=0 in its config)
3. Run the project via dbt run
4. Observe the log weirdness

Relevant log output

(venv) quigleymalcolm@Quigley-Malcolm microbatch_demo % dbt run                               
18:10:03  Running with dbt=1.10.0-a1
18:10:03  Registered adapter: postgres=1.9.0-rc1
18:10:03  Found 3 models, 429 macros
18:10:03  
18:10:03  Concurrency: 1 threads (target='dev')
18:10:03  
18:10:03  1 of 3 START sql table model microbatch_demo.input_model ....................... [RUN]
18:10:03  1 of 3 OK created sql table model microbatch_demo.input_model .................. [SELECT 4 in 0.08s]
18:10:03  2 of 3 START sql microbatch model microbatch_demo.microbatch_model ............. [RUN]
18:10:03  Batch 1 of 1 START batch 2024-12-09 of microbatch_demo.microbatch_model .............. [RUN]
18:10:03  Batch 1 of 1 OK created batch 2024-12-09 of microbatch_demo.microbatch_model ......... [MERGE 0 in 0.09s]
18:10:03  Unhandled error while executing 
1
18:10:03  2 of 3 OK created sql microbatch model microbatch_demo.microbatch_model ........ [SUCCESS in 0.12s]
18:10:03  3 of 3 START sql view model microbatch_demo.dependent_model .................... [RUN]
18:10:03  3 of 3 OK created sql view model microbatch_demo.dependent_model ............... [CREATE VIEW in 0.05s]
18:10:03  
18:10:03  Finished running 1 incremental model, 1 table model, 1 view model in 0 hours 0 minutes and 0.39 seconds (0.39s).
18:10:03  
18:10:03  Completed successfully
18:10:03  
18:10:03  Done. PASS=3 WARN=0 ERROR=0 SKIP=0 TOTAL=3

Environment

- OS: macOS 15.1
- Python: 3.10.15
- dbt branch: 1.9.latest (and also main)

Which database adapter are you using with dbt?

postgres

Additional Context