Display empty mempool instead of no mempool blocks at all.

frontend/src/app/components/mempool-blocks/mempool-blocks.component.scss

@@ -116,3 +116,13 @@
   left: 0;
   z-index: 10;
 }
+
+.empty-block {
+  width: 125px;
+  height: 125px;
+
+  right: 40px;
+  text-align: center;
+  display: flex;
+  align-items: center;
+}