Fix getResourceIdFromVersionedFilename handling empty string in query

1. fixed the issue with empty string (resource folder storage location) in the sql query

Test Plan

1. Fix for empty string in sql (overwriting workspaces and other resources)
   • copy one of the workspaces (from the resource folder) to some other place
   • open it in a notepad and change the name (don't change the filename!)
   • open Krita -> Resource Manager, make sure the Workspaces are selected and that you see the old name of the workspace
   • try to import the workspace that you copied over and edited
   • press "OK" when it asks you whether you want to overwrite it
   • the name in the Resource Manager should change to the new name

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.