MDL-64099 message: last message container minimum size

* empty last message containers should have a minimum size
to ensure list-group-item element are in the right place.

message/templates/message_drawer_conversations_list.mustache

@@ -65,7 +65,7 @@
             {{#subname}}
                 <p class="m-0 text-truncate">{{.}}</p>
             {{/subname}}
-            <p class="m-0 font-weight-light text-truncate" data-region="last-message">
+            <p class="m-0 font-weight-light text-truncate last-message" data-region="last-message">
                 {{#sentfromcurrentuser}}
                     {{#str}}
                         conversationlastmessage,

theme/boost/scss/moodle/message.scss

@@ -1345,6 +1345,9 @@ $message-drawer-width: 320px;
             border-bottom: 0;
         }
     }
+    .last-message {
+        min-height: 1.5rem;
+    }
     .section {
         .collapse {
             overflow-y: auto;

theme/boost/style/moodle.css

@@ -14201,6 +14201,8 @@ a.ygtvspacer:hover {
       border-top: 0; }
     .message-drawer .list-group .list-group-item:last-child {
       border-bottom: 0; }
+  .message-drawer .last-message {
+    min-height: 1.5rem; }
   .message-drawer .section .collapse {
     overflow-y: auto; }
   .message-drawer .section .collapsing {

theme/bootstrapbase/less/moodle/message.less

@@ -1353,6 +1353,9 @@
             }
         }
     }
+    .last-message {
+        min-height: 20px;
+    }
 }
 
 @media (max-width: 480px) {

theme/bootstrapbase/style/moodle.css

@@ -9245,6 +9245,9 @@ a.ygtvspacer:hover {
   border-bottom: 0;
   margin-bottom: 0;
 }
+.message-drawer .last-message {
+  min-height: 20px;
+}
 @media (max-width: 480px) {
   .messaging-area-container .messaging-area .messages-area.editing .messages-header {
     height: 80px;