[Backport] fix issues #4647 - drag empty sprint

app/coffee/modules/backlog/sortable.coffee

@@ -45,7 +45,7 @@ BacklogSortableDirective = () ->
 
             initIsBacklog = false
 
-            drake = dragula([$el[0], $('.empty-backlog')[0]], {
+            drake = dragula([$el[0], $('.js-empty-backlog')[0]], {
                 copySortSource: false,
                 copy: false,
                 isContainer: (el) -> return el.classList.contains('sprint-table'),
@@ -76,7 +76,8 @@ BacklogSortableDirective = () ->
                 $('.doom-line').remove()
 
                 parent = $(item).parent()
-                isBacklog = parent.hasClass('backlog-table-body') || parent.hasClass('empty-backlog')
+
+                isBacklog = parent.hasClass('backlog-table-body') || parent.hasClass('js-empty-backlog')
 
                 if initIsBacklog || isBacklog
                     sameContainer = (initIsBacklog == isBacklog)

app/partials/backlog/backlog.jade

@@ -78,7 +78,7 @@ div.wrapper(tg-backlog, ng-controller="BacklogController as ctrl",
         section.backlog-table(ng-class="{'hidden': !userstories.length}")
             include ../includes/modules/backlog-table
 
-        div.empty-large(ng-class="{'hidden': userstories === undefined || userstories.length}")
+        div.empty-large.js-empty-backlog(ng-class="{'hidden': userstories === undefined || userstories.length}")
             img(
                 src="/#{v}/images/empty/empty_mex.png"
                 alt="{{'BACKLOG.EMPTY' | translate}}"

app/styles/modules/backlog/backlog-table.scss

@@ -284,3 +284,10 @@
         }
     }
 }
+
+// drag & drop
+.empty-large {
+    .row {
+        display: none;
+    }
+}

app/styles/modules/backlog/sprints.scss

@@ -135,6 +135,7 @@
         }
     }
     .sprint-table {
+        min-height: 2rem; // drag & drop
         .sprint-empty {
             @include font-type(light);
             background: darken($whitish, 5%);