core/dbus-util: several cleanups for bus_read_mount_options()

core/dbus-util: several cleanups for bus_read_mount_options()

* Make sure ret_options is initialized on success.
* Return empty mount options as-is rather than NULL-ing it -
  dbus property parser for RootImageOptions relies on it
  for resetting options for a specific partition designator.
* Format partition:options properly with strextendf, currently
  multiple ":" will be emitted.
* Allow separator to be unset if in_out_format_str is not
  needed.