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

[codecov[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/pySTEPS/pysteps/pull/411/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pySTEPS) | Coverage Δ | | |---|---|---| | [unit_tests](https://app.codecov.io/gh/pySTEPS/pysteps/pull/411/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pySTEPS) | `83.88% <92.30%> (+0.06%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pySTEPS#carryforward-flags-in-the-pull-request-comment) to find out more.

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[RubenImhoff]

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