ImageMagick version 7.0.7-32 has known bugs that prevent it from working
ImageMagick version 7.0.7-32 generates blank previews.
There are more recent releases of ImageMagick available: https://imagemagick.org/script/changelog.php
I've built 7.0.7-39 and it works fine, the bug is gone.
I've tried building latest 7.0.8-16, but then php7-imagick throws warnings that it was built against an older version of ImageMagick and that may cause errors.