Fix returning possible null values

[dkreuer]

The function may return null values but the return type annotation restricts to return strings only. This workaround prevents PHP from crashing by returning an empty string instead - which is fine because the result is sent to output.

Supersedes PR #183 (now targets v3.x branch as it is a bugfix).

[codecov-io]

Codecov Report

Merging #184 into v3.x will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               v3.x     #184   +/-   ##
=========================================
  Coverage     92.05%   92.05%           
  Complexity      381      381           
=========================================
  Files            56       56           
  Lines          1272     1272           
=========================================
  Hits           1171     1171           
  Misses          101      101           

Impacted Files	Coverage Δ	Complexity Δ
Command/ListClientsCommand.php	96.72% <100.00%> (ø)	7.00 <0.00> (ø)

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[dkreuer]

Will fix the failing (added) test tomorrow.

[X-Coder264]

@dkreuer Thanks for the PR.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.05%. Comparing base (6d65806) to head (bbbf5ed).
Report is 87 commits behind head on v3.x.

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Additional details and impacted files

@@            Coverage Diff            @@
##               v3.x     #184   +/-   ##
=========================================
  Coverage     92.05%   92.05%           
  Complexity      381      381           
=========================================
  Files            56       56           
  Lines          1272     1272           
=========================================
  Hits           1171     1171           
  Misses          101      101           

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