We've kludged around this for now with the following in our cas.css overlay:
.banner-danger:empty {
border-color: transparent !important;
background-color: transparent !important;
box-shadow: none !important;
padding: 0 !important;
margin: 0 !important;
}
We probably need to review changes to the 7.3 dist cas.css that have accrued since 7.0.