fix: multiple calls of dbt-retry fail with exit code 2

[showyou]

Resolves #8848

Problem

When calling dbt retry multiple times, if the previous run returned zero results and completed successfully, the next run resulted in an error.

The cause is as described in the issue, but to put it simply: when the count of results in the ExecutionResult class is 0, not self.previous_results evaluates to True in dbt/core/dbt/task/retry.py, leading to unexpected results.

self.previous_results = load_result_state(
    Path(config.project_root) / Path(state_path) / RUN_RESULTS_FILE_NAME
        )
if not self.previous_results:
    raise DbtRuntimeError(

Solution

if not self.previous_results:

have been changed to

if self.previous_results is none:

I also added test class about this issue.

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 (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

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