Fix external nowcast only zeros

[mats-knmi]

I thought I fixed this in #546, but I realized now that an external nowcast does not necessarily have to have nan's at the edge (especially in our usecase where we run the external nowcast on a larger domain to explicitly prevent this).

This should not be an issue if it were not for the bug where, if there are no nan's anywhere in the domain, the code that is supposed to fill this part with values from nwp will simply set the entire domain to 0.

The simple fix is to not try to fill in the nan's when there are no nan's anywhere in the domain.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.64%. Comparing base (45824f2) to head (7e239da).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #566   +/-   ##
=======================================
  Coverage   84.64%   84.64%           
=======================================
  Files         170      170           
  Lines       15095    15095           
=======================================
  Hits        12777    12777           
  Misses       2318     2318           

Flag	Coverage Δ
unit_tests	84.64% <100.00%> (ø)

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