fix: issue with example code

[mats-knmi]

Fixes #548

Looks like if you pass it a nowcast with only a single member the nan mask will not have the correct shape at the time it is used. This is because a single member gets repeated to match the number of members expected. Now the same happens for the nan mask.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.17%. Comparing base (aa8de93) to head (46ba953).

Files with missing lines	Patch %	Lines
pysteps/blending/steps.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #549      +/-   ##
==========================================
- Coverage   84.18%   84.17%   -0.01%     
==========================================
  Files         170      170              
  Lines       14984    14985       +1     
==========================================
  Hits        12614    12614              
- Misses       2370     2371       +1     

Flag	Coverage Δ
unit_tests	84.17% <0.00%> (-0.01%)	⬇️

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[dnerini]

thanks @mats-knmi, I can confirm that the fix solves the regression described in #548

edit: I'm a bit concerned we didn't capture this with our tests, should we include this test case too?