Bugfix: Added a check for file existance to prevent tag scanning errors.

Summary:
It is a workaround for bug T18572. At least it helps in
writing operations if the file does not exist.

Test Plan: code review & application test

Reviewers: O1 MITK Reviewer Group I, kislinsk

Reviewed By: O1 MITK Reviewer Group I, kislinsk

Differential Revision: https://phabricator.mitk.org/D427