Fix accessing uncached root group variable when using NetCDF4FileHandler in caching mode

[gerritholl]

In the NetCDF4FileHandler, when the file handler is being cached, accessing a non-cached variable in the root group was failing with a TypeError because None is being passed to os.fspath.
This PR adds a (regression) unit test for this problem and a fix so that the problem is avoided.

• Closes Fix accessing uncached root group variable when using NetCDF4FileHandler in caching mode #1195
• Tests added
• Tests passed
• Passes flake8 satpy

[djhoese]

LGTM

[coveralls]

Coverage increased (+0.001%) to 89.622% when pulling 6c34b6c on gerritholl:fix-root-group-variable-cached-netcdf-utils into cf9aab4 on pytroll:master.

[codecov]

Codecov Report

Merging #1195 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1195   +/-   ##
=======================================
  Coverage   89.62%   89.62%           
=======================================
  Files         200      200           
  Lines       29550    29553    +3     
=======================================
+ Hits        26483    26486    +3     
  Misses       3067     3067           

Impacted Files	Coverage Δ
satpy/readers/netcdf_utils.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_netcdf_utils.py	95.40% <100.00%> (+0.05%)	⬆️

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