Fix support for timesteps list in case model has rain but radar does not

[mats-knmi]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.88%. Comparing base (07a5aa8) to head (39d8c0d).

Files	Patch %	Lines
pysteps/blending/steps.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #411      +/-   ##
==========================================
+ Coverage   83.82%   83.88%   +0.06%     
==========================================
  Files         160      160              
  Lines       12788    12785       -3     
==========================================
+ Hits        10719    10725       +6     
+ Misses       2069     2060       -9     

Flag	Coverage Δ
unit_tests	83.88% <92.30%> (+0.06%)	⬆️

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[RubenImhoff]

Thanks for this extra fix related to #368, @mats-knmi!