[Studio] fix: render zero throughput without visible bars

[yx9o]

What is the purpose of the change

Fixes #628 .

Prevent MiniBar from rendering zero throughput values as visible 4px bars, which can make idle periods appear active.

Brief changelog

• Render zero values with 0px height.
• Preserve the minimum 4px height for positive values.
• Add regression tests for all-zero and mixed throughput data.