Saving of empty credentials blocks launch of Asset Discovery

Issue Summary

After some manipulation with credentials tab in settings mode, Discovery cannot be launched anymore.

Steps to Reproduce

1. Run Discovery in settings mode
2. Add 1 Scan Setting with any IP (e.g. 127.0.0.1)
3. Add credentials with any type (e.g. Windows). Leave Login and Password fields empty
4. Close Discovery to Save settings
5. Run Discovery in settings mode
6. Remove Scan Settings, Credentials
7. Close Discovery to Save settings
8. Run Discovery in settings mode

Expected Results

Discovery runs without any errors

Actual Results

Discovery don't runs.

The below exception is thrown in logs:

Old encrypt algorithm 

Workaround

Remove credentialstore file from Discovery folder