clutter/frame-clock: Schedule a frame at least once per second

Users of Debian arm64 (aarch64) report that on at least some GPUs or screens, after time-based screen blanking has occurred, it is not possible to unlock the screen. Bisection indicates that this regressed in commit 209b1ba3, so presumably this is because a refresh rate of 0 is reported while the screen is blanked, leading to the frame clock pausing forever.

Fixes: 209b1ba3 "clutter/frame-clock: Adapt refresh rate from to frame info"
Closes: #1536 (closed)
Bug-Debian: https://bugs.debian.org/974172