need better handling of empty chunks when using --txs

[sslivkoff]

gracefully handle empty chunks, currently panics due to empty mpsc buffer

thread 'thread 'tokio-runtime-workertokio-runtime-worker' panicked at '' panicked at 'mpsc bounded channel requires buffer > 0mpsc bounded channel requires buffer > 0', ', crates/freeze/src/datasets/state_diffs.rs:159:28
crates/freeze/src/datasets/state_diffs.rs:159note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
:28
thread 'tokio-runtime-worker' panicked at 'mpsc bounded channel requires buffer > 0', crates/freeze/src/datasets/state_diffs.rs:159:28
thread 'thread 'tokio-runtime-worker' panicked at 'mpsc bounded channel requires buffer > 0', crates/freeze/src/datasets/state_diffs.rs:159:tokio-runtime-worker' panicked at '28mpsc bounded channel requires buffer > 0
', crates/freeze/src/datasets/state_diffs.rs:159:28
thread 'tokio-runtime-worker' panicked at 'mpsc bounded channel requires buffer > 0', crates/freeze/src/datasets/state_diffs.rs:159:28
thread 'thread 'thread 'tokio-runtime-workertokio-runtime-worker' panicked at 'tokio-runtime-workermpsc bounded channel requires buffer > 0' panicked at '' panicked at 'mpsc bounded channel requires buffer > 0mpsc bounded channel requires buffer > 0', ', ', crates/freeze/src/datasets/state_diffs.rscrates/freeze/src/datasets/state_diffs.rscrates/freeze/src/datasets/state_diffs.rs::159:15928:
:159:28
28
thread 'tokio-runtime-worker' panicked at 'mpsc bounded channel requires buffer > 0', crates/freeze/src/datasets/state_diffs.rs:159:28
...done