Add None to typing of timestep_start_full_nwp_weight

[RubenImhoff]

Minor fix to the typing in timestep_start_full_nwp_weight in ./blending/steps.py.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.89%. Comparing base (1367ab4) to head (6e2a9ac).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #541   +/-   ##
=======================================
  Coverage   83.89%   83.89%           
=======================================
  Files         168      168           
  Lines       14678    14678           
=======================================
  Hits        12314    12314           
  Misses       2364     2364           

Flag	Coverage Δ
unit_tests	83.89% <100.00%> (ø)

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