Bugfix : Fix extension for saving reference image set

This MR add krf as extension if the extension is empty by default. Now the collection is save as name. and cannot be loaded later since krita can see only krf files .

Test Plan

• Add Reference Images
• Export Reference Image Set
• Select "All supported formats" and uncheck "automatically select .krf"
• Save the collection and verify it is saved as name.krf instead of "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.