[plugin-ssh] Avoid using empty string as result for SFTP commands not providing any result

[valfirst]

Closes #2843

[codecov]

Codecov Report

Merging #3662 (3301a6c) into master (a6b6f7c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #3662   +/-   ##
=========================================
  Coverage     96.97%   96.97%           
- Complexity     6094     6097    +3     
=========================================
  Files           860      860           
  Lines         17447    17450    +3     
  Branches       1137     1137           
=========================================
+ Hits          16920    16923    +3     
  Misses          422      422           
  Partials        105      105           

Impacted Files	Coverage Δ
...src/main/java/org/vividus/ssh/sftp/SftpOutput.java	100.00% <ø> (ø)
...in-ssh/src/main/java/org/vividus/ssh/SshSteps.java	100.00% <100.00%> (ø)
...c/main/java/org/vividus/ssh/sftp/SftpExecutor.java	100.00% <100.00%> (ø)
...java/org/vividus/ssh/sftp/SftpOutputPublisher.java	100.00% <100.00%> (ø)

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication