Time panel does not show density when only timeless data is present

[jleibs]

We resolved:

• Nothing is displayed when only timeless data has been logged #1175

By ensuring the log_time timeline is always created:

• Always create the log_time timeline #1763

However, the time panel still does not behave correctly. The TimeAxis code path makes a couple of assertions that we must have a non-empty set (

rerun/crates/re_viewer/src/ui/time_panel/time_axis.rs

Line 54 in 9a07e4e

assert!(!times.is_empty());

), so initializing that with a default empty set as done in #1763 does not seem to be the correct solution.

The timeless data is being added separately to the density graph (

rerun/crates/re_viewer/src/ui/time_panel/data_density_graph.rs

Line 433 in 9a07e4e

add_data_point(TimeRange::point(TimeInt::BEGINNING), num_timeless_messages);

) , but it seems something is still messed up in how the range is determined.