Handling edge cases in speed averaging

[robfitzgerald]

As shown in #88, a run of bambam-omf can panic when no speeds are found on any segments in the downloaded region:

 RUST_LOG=info ./rust/target/release/bambam_omf network -c configuration/bambam-omf/travel-mode-filter.json -b -105.173256,-105.164802,39.817799,39.823534 -o out/test                                   
[2026-01-26T22:26:36Z INFO  bambam_omf::app::network] running OMF import with
            object store AmazonS3
            rg_chunk_size 4
            file_concurrency_limit 64
            release latest
            (xmin,xmax,ymin,ymax): -105.173256,-105.1648,39.8178,39.823532
[2026-01-26T22:26:37Z INFO  bambam_omf::collection::collector] Collecting OvertureMaps Connector records from release 2026-01-21.0
[2026-01-26T22:26:38Z INFO  bambam_omf::collection::collector] Started collection
[2026-01-26T22:26:47Z INFO  bambam_omf::collection::collector] Collection time 8.999544084s
[2026-01-26T22:26:47Z INFO  bambam_omf::collection::collector] Deserialization time 1.1105ms
[2026-01-26T22:26:47Z INFO  bambam_omf::collection::collector] Total time 9.000756292s
[2026-01-26T22:26:47Z INFO  bambam_omf::collection::collector] Collecting OvertureMaps Segment records from release 2026-01-21.0
[2026-01-26T22:26:48Z INFO  bambam_omf::collection::collector] Started collection
[2026-01-26T22:26:58Z INFO  bambam_omf::collection::collector] Collection time 10.015029208s
[2026-01-26T22:26:58Z INFO  bambam_omf::collection::collector] Deserialization time 452.75µs
[2026-01-26T22:26:58Z INFO  bambam_omf::collection::collector] Total time 10.015506542s

[2026-01-26T22:26:58Z INFO  bambam_omf::graph::serialize_ops] all connectors accounted for

thread 'main' panicked at bambam-omf/src/bin/bambam_omf.rs:7:19:
called `Result::unwrap()` on an `Err` value: InternalError("internal division by zero when computing average speed: [None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None]")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This points to two issues:

• speed averaging is semigroupal in nature; if there are no values, there is no mean speed, so it fails to be a good infill for missing speed values. In that case, we should probably alert the command line and skip building the speed file
• speed averaging should also short circuit if there are no speed values, and the function computing average speeds should return None when there are no speeds to average

[robfitzgerald]

closed via #95.