Revenue pool: define `batch_distribute` behavior for empty input vector

[greatest0fallt1me]

Description

Choose and implement either no-op or panic for empty payments; document rationale; align with vault batch policies if applicable.

Requirements and context

• Must be easy for admin tooling to reason about.
• Add tests proving behavior.

Suggested execution

• Fork the repo and create a branch: git checkout -b test/revenue-pool-batch-empty
• Implement changes in the Callora-Contracts Soroban workspace (contracts/*/src/).
• Run cargo fmt, cargo clippy --all-targets --all-features -- -D warnings, and cargo test from the workspace root.
• For WASM release builds: cargo build --target wasm32-unknown-unknown --release -p callora-revenue-pool (adjust -p if the crate name differs).

Deliverables

• lib.rs + test.rs + short doc note.

Test and commit

• Run tests and ./scripts/coverage.sh (or cargo tarpaulin per tarpaulin.toml).
• Cover edge cases; include summarized test output and brief security notes in the PR description.

Example commit message

test(revenue-pool): define empty batch_distribute behavior

Guidelines

• Minimum 95% line coverage for touched crates (workspace policy).
• Clear documentation (Rust /// on public items where applicable; repo markdown as needed).
• Timeframe: 96 hours from assignment unless agreed otherwise.

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