evp/tests: Test empty lines

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
libeventd/nd-notification: Ignore empty message

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
nd/cairo: Vertically center title when no message

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>