[sddm-theme] Don't handle enter key when blurred

Summary:
Meaning pressing enter just wakes up the display.
Behaviour now matches lock screen.

BUG: 402957
Test Plan: Ran SDDM

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17997