Always ensure_unicode for subprocess output

[nmuesch]

What does this PR do?

Remove unnecessary if branches for ensuring unicode around the output from subprocess.

Motivation

If the response from subprocess is empty, python3 will return a byte and not be converted to unicode, which can break some checks that don't require subproccess_out returns a non empty value.

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo
• If PR adds a configuration option, it has been added to the configuration file.

Additional Notes

Anything else we should know when reviewing?

[codecov-io]

Codecov Report

Merging #2941 into master will decrease coverage by 9.16%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2941      +/-   ##
==========================================
- Coverage    84.7%   75.54%   -9.17%     
==========================================
  Files         659       45     -614     
  Lines       37508     3570   -33938     
  Branches     4482      427    -4055     
==========================================
- Hits        31772     2697   -29075     
+ Misses       4421      764    -3657     
+ Partials     1315      109    -1206

[zippolyte]

Do we still need to check for None ?

[nmuesch]

Yeah, lets. The subprocess command seems to actually come from some binding in the Agent, so better to be safer than sorry here.