Fix EWA resampling when the result is all fill values

[djhoese]

This was a bug I discovered at the same time I found #342, but had a really difficult time reproducing. The main piece of code that is effected is only called when dask determines that it should call the "aggregate" function of the reduction with a specific result from the previous "combination" calls. I was able to force this by making the input one single chunk.

• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[codecov]

Codecov Report

Merging #343 (7f5f272) into master (30422e6) will increase coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head 7f5f272 differs from pull request most recent head 8a5e1ee. Consider uploading reports for the commit 8a5e1ee to get more accurate results

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
+ Coverage   92.99%   93.03%   +0.04%     
==========================================
  Files          48       48              
  Lines        9798     9813      +15     
==========================================
+ Hits         9112     9130      +18     
+ Misses        686      683       -3     

Flag	Coverage Δ
unittests	93.03% <100.00%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
pyresample/ewa/dask_ewa.py	90.71% <100.00%> (+1.26%)	⬆️
pyresample/test/test_dask_ewa.py	98.84% <100.00%> (+0.10%)	⬆️

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[coveralls]

Coverage increased (+0.04%) to 93.029% when pulling 8a5e1ee on djhoese:bugfix-dask-ewa-empty-chunk into 30422e6 on pytroll:master.

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[mraspaud]

LGTM