fix plot_predictions if no median forecast available

[sbfnk]

At the moment plot_predictions throws an error if forecasts do not contain a median. This fixes that.

[codecov]

Codecov Report

Merging #137 (4cdaf9f) into master (6895b49) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

❗ Current head 4cdaf9f differs from pull request most recent head 177414c. Consider uploading reports for the commit 177414c to get more accurate results

@@            Coverage Diff             @@
##           master     #137      +/-   ##
==========================================
- Coverage   48.40%   48.36%   -0.04%     
==========================================
  Files          18       18              
  Lines        1345     1346       +1     
==========================================
  Hits          651      651              
- Misses        694      695       +1     

Impacted Files	Coverage Δ
R/plot.R	0.00% <0.00%> (ø)

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[nikosbosse]

👍