Skip creating the exported xml translation files entirely if they would
be empty (not having any rows).

Adjust the ReducedTranslactionExportTests to test the modified behavior
- Rename test expectedEmptyTranslationFiles to expectedEmptyTranslationFilesAbsent
  as now no 'empty (just 2 header lines) xml file is expected when there
  is no content but instead the xml file should be absent/missing
- Adjust testExistInFullAndNotExistsInReduced testcase (which checks that
  a specific text is absent) to allow for the reduced file to be missing
  - That way the test case can be used flexibly for checking the
    'reduced export' functionality without needing to know if the
    resulting xml file got empty/missing