Substitute with isNullOrEmpty() call

[FourteenBrush]

Description

Linked issue:

none

Linked website pull request, if the documention of https://tinylog.org/v2/ has to be updated:

none

Definition of Done

• I read contributing.md
• There are no TODOs left in the code
• Code style follows the tinylog standard
• All classes and methods have Javadoc
• Changes are covered by JUnit tests including edge cases, errors, and exception handling
• Maven build works including compiling, tests, and checks (mvn verify)
• Changes are committed by a verified email address that is assigned to the GitHub account (https://github.com/settings/emails)
• Documentation on the tinylog website is up-to-date or updated by a pull request to the repository tinylog-org/website

Agreements

• I agree that my changes will be published under the terms of the Apache License 2.0 (mandatory)
• I agree that my GitHub user name will be published in the release notes (optional)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bebb475) 94.27% compared to head (820cf5f) 94.27%.

Additional details and impacted files

@@            Coverage Diff            @@
##               v2.7     #678   +/-   ##
=========================================
  Coverage     94.27%   94.27%           
  Complexity     2833     2833           
=========================================
  Files           149      149           
  Lines          5484     5484           
  Branches        725      725           
=========================================
  Hits           5170     5170           
  Misses          176      176           
  Partials        138      138           

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[pmwmedia]

Thank you!