refactor: use is_empty() instead of len() > 0 checks in perftest.rs

[reallesee]

Replace len() > 0 checks with !is_empty() in perftest.rs for better idiomatic Rust code. This change follows Rust's best practices by using the dedicated is_empty() method for checking collection emptiness, which is more readable and can be more efficient for certain collection types

[vercel]

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