fixes #16769: use a clearfix on the .modal-header instead of a min-height to...

fixes #16769: use a clearfix on the .modal-header instead of a min-height to avoid collapsing when only a dismiss button is available and no .modal-title

less/modals.less

@@ -79,7 +79,7 @@
 .modal-header {
   padding: @modal-title-padding;
   border-bottom: 1px solid @modal-header-border-color;
-  min-height: (@modal-title-padding + @modal-title-line-height);
+  &:extend(.clearfix all);
 }
 // Close icon
 .modal-header .close {