Fix logic to disable server default plugins loading

[diegoferigo]

Summary

#281 introduced the new default behavior of loading server plugins from a configuration file. After the discussion in #281 (comment), if IGN_GAZEBO_SERVER_CONFIG is set but empty, no server plugins should loaded and the server.config file should be ignored.

I'm testing this functionality and it seems not working correctly. In fact, the return value of ignition::common::env complies with the intended logic only in the variant that accepts the _allowEmpty=true argument, which was missing.

To provide extra downstream content, I'm experimenting loading plugins with the new method introduced in #936. I'm trying to implement robotology-legacy/gym-ignition#378, but as soon as I removed the only plugin that I was loading, the server automatically reverted to the default list of plugins part of server.config. This was already known, as ticketed in robotology-legacy/gym-ignition#289.

cc @mjcarroll

Note: accordingly to the documentation, this logic will not likely work in Windows due to the its behavior with empty environment variables.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #953 (eb5e585) into ign-gazebo3 (59a2faf) will not change coverage.
The diff coverage is n/a.

❗ Current head eb5e585 differs from pull request most recent head 8ebd8f2. Consider uploading reports for the commit 8ebd8f2 to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo3     #953   +/-   ##
============================================
  Coverage        77.80%   77.80%           
============================================
  Files              220      220           
  Lines            12579    12579           
============================================
  Hits              9787     9787           
  Misses            2792     2792           

Impacted Files	Coverage Δ
src/ServerConfig.cc	90.95% <ø> (ø)

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[chapulina]

Thanks for the fix!