add nodata value to "empty array"

[roeldegoede]

Issue addressed

Fixes #

Explanation

When starting with a very small dataset, it is quite likely that da1 is an xarray full with nan values. Without a proper nodata value, we run into trouble in the merge function, hence added explicitly now.

Checklist

• Updated tests or added new tests
• Branch is up to date with main
• Updated documentation if needed
• Updated changelog.rst if needed

Additional Notes (optional)

Add any additional notes or information that may be helpful.