[reve] Fix misleading client error message in streaming of an empty projected track

[alja]

The client logs reported an error in streaming binary data from the server to the client.
The erroneous message was generated because REveTrackProjected::WriteCoreJson created a render_data JSON object even when the track had no points. This can be reproduced by creating a track with a vertex outside the track propagator radius and then projecting it.

The missing check for an empty track in REveTrackProjected::WriteCoreJson had no consequence on streaming and rendering of other elements in the scene.

[alja]

@linev Please check.

[github-actions]

Test Results

    21 files      21 suites   3d 14h 55m 20s ⏱️
 3 868 tests  3 818 ✅   0 💤 50 ❌
73 317 runs  73 061 ✅ 205 💤 51 ❌

For more details on these failures, see this check.

Results for commit 0839e3a.