Fix enhancement config loading when yaml file is empty

[djhoese]

I was playing around with making my own enhancement configuration files and noticed this small bug. If you have a completely empty enhancement config then you get an exception and nothing happens.

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

[coveralls]

Coverage decreased (-0.002%) to 69.05% when pulling fb55ea3 on djhoese:bugfix-empty-enhancement into fbe75c2 on pytroll:master.

[codecov]

Codecov Report

Merging #387 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #387      +/-   ##
==========================================
- Coverage   69.05%   69.04%   -0.01%     
==========================================
  Files         123      123              
  Lines       15710    15722      +12     
==========================================
+ Hits        10848    10856       +8     
- Misses       4862     4866       +4

Impacted Files	Coverage Δ
satpy/tests/test_writers.py	98.05% <100%> (+0.12%)	⬆️
satpy/writers/__init__.py	75.07% <100%> (+0.22%)	⬆️
satpy/readers/__init__.py	93.33% <0%> (-1.49%)	⬇️

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