`assemble_acoustics_containers()` fails ungracefully if there are no detections

[edwardlavender]

If there are no detections, assemble_acoustics_containers() fails with:

Error in `arrange()` at patter/R/assemble-data-internals.R:156:3:
ℹ In argument: `..2 = .data$sensor_id`.
Caused by error in `.data$sensor_id`:
! Column `sensor_id` not found in `.data`.
Run `rlang::last_trace()` to see where the error occurred.

assemble_acoustics_containers() should return an empty data.table or NULL instead.

[edwardlavender]

The same error is produced if only one detection.