Gif showing the problem.
https://imgur.com/W19AYv7.gif

1) Open the sample scene
2) Generate lighting
3) Goto lightmap debug scene mode (its easier to view the problem here)
4) See probes are not white despite strong sun lighting
5) Select Assets/Meshes/box01 import settings
6) Click Generate Lightmap UVs
7) Click Apply
8) Generate lighting
9) Observe that lightprobes are now correctly lit

Observed: Lightprobes are not correctly lit when contributing meshes do not have lightmapping UVs
Expected: Lightprobes are correctly lit with contributing meshes regardless of UVs

Notes: Reproduces on CPU and GPU Lightmappers.

Seen in 2021.2.0a18

Found by @julien while working on APV: see thread here: https://unity.slack.com/archives/C06TQ5M4J/p1621266271376200