Fix rank picture mode without matrix

[avinxshKD]

What type of PR is this?

/kind bug

What this PR does / why we need it:

selected_and_all_and_picture currently assumes every result has a Matrix metric.

For normal benchmarks with scalar metrics only, _draw_pictures() gets None and crashes on matrix.keys(), even though the rank CSV files can be saved.

Which issue(s) this PR fixes:

Fixes #559

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[gemini-code-assist]

Code Review

This pull request adds a safety check in core/storymanager/rank/rank.py to verify if the matrix object is valid before attempting to iterate over its keys, preventing potential errors. There are no review comments, and I have no additional feedback to provide.

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[avinxshKD]

pls take a look, minimal fix @MooreZheng @hsj576