Do not set advanced options with empty string to true

[Jojo-1000]

There are advanced options such as force-local that expect an empty string in some cases. While saving, these were replaced with true even though there was an equals sign. Now only options without an equals sign are set to true.

The options were not set to true when opening the configuration but already when saving, so the saved backup configuration was different than in the UI.

Closes #4757

Steps to reproduce

1. Make a backup set in the GUI
2. set the force-locale option to blank string
3. save, close and re-open the config page of that backup
4. see that the blank field was filled with true
5. run the backup
6. warning that true is not a valid locale appears

• Actual result:
  Option automatically set to true.
  Backup throws warning
• Expected result:
  Option should remain blank.
  Backup should complete without warning.

Old behavior

Set flags and empty values to true before saving:
--dry-run => --dry-run=true
--force-locale= => --force-locale=true

New behavior

Set flags to true (as before):
--dry-run => --dry-run=true

Keep empty options with empty value:
--force-locale= => --force-locale=