Fix a corner case in region-aggregation with missing data

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• ~Documentation Added~
• ~Name of contributors Added to AUTHORS.rst~
• [x] Description in RELEASE_NOTES.md Added

Description of PR

This PR fixes a corner-case in the region-aggregation-with weights if all data for a year are missing.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (7aa73eb) 94.8% compared to head (d3fef85) 94.8%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #813 +/- ## ======================================= - Coverage 94.8% 94.8% -0.1% ======================================= Files 62 62 Lines 6042 6040 -2 ======================================= - Hits 5729 5727 -2 Misses 313 313 ```

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