[nightcolor] Fix division by zero

Summary:
If qAbs(targetTemp - m_currentTemp) is less than TEMPERATURE_STEP, then
it will result in a division by zero.

FIXED-IN: 5.17.5
BUG: 415359

Reviewers: KWin, apol

Reviewed By: apol

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D26493