text: Title: `--legacy-output` does not work with Rebot when combining, merging or filtering results
  Body: Because robot scripts are not always stable, due to infrastructural problems, we use a script to perform failing tests with a maximum of 3 times. Until tests are run ok... or eventually fail.
  
  This will generate several output.xml which are combined into a single output.xml (report.html and log.html) using rebot.
  Until Robotframework v6.1.1 this was working fine. I started to use version 7.0 and used the option --legacyoutput with both robot and rebot.
  
  When I combine MORE THEN one output,xl, rebot will fail:
  `
  rebot --legacyoutput --merge --outputdir <some outputdir> --output output.xml firstoutput.xml secondoutput.xml
  
  [ ERROR ] Unexpected error: AttributeError: 'NoneType' object has no attribute 'isoformat'
  Traceback (most recent call last):
    File "C:\Python311\Lib\site-packages\robot\utils\application.py", line 81, in _execute
      rc = self.main(arguments, **options)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\Python311\Lib\site-packages\robot\rebot.py", line 340, in main
      rc = ResultWriter(*datasources).write_results(settings)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\Python311\Lib\site-packages\robot\reporting\resultwriter.py", line 57, in write_results
      self._write_output(results.result, settings.output, settings.legacy_output)
    File "C:\Python311\Lib\site-packages\robot\reporting\resultwriter.py", line 71, in _write_output
      self._write('Output', result.save, path, legacy_output)
    File "C:\Python311\Lib\site-packages\robot\reporting\resultwriter.py", line 84, in _write
      writer(path, *args)
    File "C:\Python311\Lib\site-packages\robot\result\executionresult.py", line 154, in save
      self.visit(writer(target, rpa=self.rpa))
    File "C:\Python311\Lib\site-packages\robot\result\executionresult.py", line 168, in visit
      visitor.visit_result(self)
    File "C:\Python311\Lib\site-packages\robot\result\visitor.py", line 44, in visit_result
      result.suite.visit(self)
    File "C:\Python311\Lib\site-packages\robot\model\testsuite.py", line 420, in visit
      visitor.visit_suite(self)
    File "C:\Python311\Lib\site-packages\robot\model\visitor.py", line 131, in visit_suite
      suite.suites.visit(self)
    File "C:\Python311\Lib\site-packages\robot\model\itemlist.py", line 102, in visit
      item.visit(visitor)    # type: ignore
      ^^^^^^^^^^^^^^^^^^^
    File "C:\Python311\Lib\site-packages\robot\model\testsuite.py", line 420, in visit
      visitor.visit_suite(self)
    File "C:\Python311\Lib\site-packages\robot\model\visitor.py", line 131, in visit_suite
      suite.suites.visit(self)
    File "C:\Python311\Lib\site-packages\robot\model\itemlist.py", line 102, in visit
      item.visit(visitor)    # type: ignore
      ^^^^^^^^^^^^^^^^^^^
    File "C:\Python311\Lib\site-packages\robot\model\testsuite.py", line 420, in visit
      visitor.visit_suite(self)
    File "C:\Python311\Lib\site-packages\robot\model\visitor.py", line 131, in visit_suite
      suite.suites.visit(self)
    File "C:\Python311\Lib\site-packages\robot\model\itemlist.py", line 102, in visit
      item.visit(visitor)    # type: ignore
      ^^^^^^^^^^^^^^^^^^^
    File "C:\Python311\Lib\site-packages\robot\model\testsuite.py", line 420, in visit
      visitor.visit_suite(self)
    File "C:\Python311\Lib\site-packages\robot\model\visitor.py", line 135, in visit_suite
      self.end_suite(suite)
    File "C:\Python311\Lib\site-packages\robot\output\xmllogger.py", line 375, in end_suite
      self._write_status(suite)
    File "C:\Python311\Lib\site-packages\robot\output\xmllogger.py", line 444, in _write_status
      'starttime': self._datetime_to_timestamp(item.start_time),
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\Python311\Lib\site-packages\robot\output\xmllogger.py", line 432, in _datetime_to_timestamp
      return dt.isoformat(' ', timespec='milliseconds').replace('-', '')
             ^^^^^^^^^^^^
  AttributeError: 'NoneType' object has no attribute 'isoformat'
  `
  
  Is it something I'm doing wrong ? I found it confusing that the documentation & release notes mention the option "--legacy-output", but both robot.exe --help and rebot --help, only have the option "--legacyoutput" (without -). 