Fix benchmark function to safely access max_pass_run_data and no successful runs

[skalthoff]

This pull request enhances error handling and robustness in the benchmark function of jellybench_py/core.py. The main adjustment ensures dictionary keys are safely accessed, preventing runtime errors caused by missing keys.

Previously, encountering missing keys (speed or rss_kb) in the max_pass_run_data dictionary triggered a KeyError, interrupting execution:

KeyError: 'speed'

Improvements:

• Updated dictionary key access within the benchmark function
• Implemented the get method with appropriate default values (0.0 for speed, and 0 for rss_kb) to gracefully handle missing keys.

These changes isolate the fix within the affected function, enhancing code stability without side effects elsewhere.

[BotBlake]

Since we only talked about this PR in private until now, let me comment on GitHub as well.
Implementation-wise it seems to do the same as #87
However I am not a fan of that implementation approach.

Specifically I want to understand WHY these values could be missing.
Since these values are IMPORTANT for the test results, I personaly do not think it is a good idea to have them be anything BUT the values provided directly from ffmpeg.

In my oppinion a test where ANY ffmpeg process does not return all requied values should be marked as FAILED, with a failure reason that explains the situation. (e.g. "missing_values")

Perhaps change the PR as a Draft if you intend to work on this.
Otherwise I suggest closing it for the time being and re-open it when necessary.