Check for empty result in Modify shell files

[ilija-lazoroski]

What does this PR do?

Fixes #1507 and #1055 .

In docker container /etc/passwd doesn't have any regular users. The modify shell script searches for those kind of users and if it doesn't find it return empty list which then we send telemetry without any result making the post breach action to fail.
Here we check if we have empty result from the running of command and send failed result with explanation.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by {Running the Monkey locally with relevant config/running Island/...}

• If applicable, add screenshots or log transcripts of the feature working

Explain Changes

[codecov]

Codecov Report

Merging #1510 (1883ecd) into develop (0a4973a) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

❗ Current head 1883ecd differs from pull request most recent head dc1735e. Consider uploading reports for the commit dc1735e to get more accurate results

@@             Coverage Diff             @@
##           develop    #1510      +/-   ##
===========================================
- Coverage    43.00%   42.98%   -0.02%     
===========================================
  Files          477      477              
  Lines        14171    14176       +5     
===========================================
  Hits          6094     6094              
- Misses        8077     8082       +5     

Impacted Files	Coverage Δ
.../post_breach/actions/modify_shell_startup_files.py	0.00% <0.00%> (ø)
monkey/infection_monkey/system_info/__init__.py	41.26% <0.00%> (-1.36%)	⬇️
...ction_monkey/system_info/windows_info_collector.py	0.00% <0.00%> (ø)

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[VakarisZ]

Approved, provided you tested this and actually get this output in the report

[shreyamalviya]

Minor rewording

[mssalvatore]

Changelog entries should be meaningful to the user of the changed component. "Malfunction" is a little bit broad, and the issue isn't specifically docker related.

Suggested change

	- Malfunctioning Modify shell startup files PBA on Docker. #1507
	- Unhandled error when "modify shell startup files PBA" is unable to find regular users. #1507