Preserve metadata when merging cubes without bands

[Silvren]

Summary

• preserve cube metadata when merge_cubes receives cubes without band dimensions
• retain the available band metadata when only one input cube has a band dimension
• keep the existing band-union behavior when both cubes have band dimensions
• keep metadata unknown when either input has no metadata

Testing

• python -m pytest tests/rest/datacube/test_datacube100.py -q (347 passed, 2 skipped)
• pre-commit run --files CHANGELOG.md openeo/rest/datacube.py tests/rest/datacube/test_datacube100.py

Fixes #783