Fixed bug that caused automatic download to fail in rare cases

[schlunma]

Description

This PR fixes a bug in the automatic download feature that led to weird error messages. It assigns median_speed = 0.0 if no non-zero values for the download speed can be found from past runs. This avoids cryptic error messages that result from using the download feature on a node without internet connection.

Closes #1440

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[codecov]

Codecov Report

Merging #1442 (609456b) into main (2d3df9d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1442   +/-   ##
=======================================
  Coverage   89.65%   89.66%           
=======================================
  Files         196      196           
  Lines       10336    10339    +3     
=======================================
+ Hits         9267     9270    +3     
  Misses       1069     1069           

Impacted Files	Coverage Δ
esmvalcore/esgf/_download.py	100.00% <100.00%> (ø)

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