Fix a content parsing bug.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/model/content.dart

@@ -990,14 +990,14 @@ class _ZulipContentParser {
       if (child is! dom.Element) return null;
       if (child.localName != 'pre') return null;
 
-      if (child.nodes.length > 2) return null;
+      if (child.nodes.length > 2 || child.nodes.isEmpty) return null;
       if (child.nodes.length == 2) {
         final first = child.nodes[0];
         if (first is! dom.Element
             || first.localName != 'span'
             || first.nodes.isNotEmpty) return null;
       }
-      final grandchild = child.nodes[child.nodes.length - 1];
+      final grandchild = child.nodes.last
       if (grandchild is! dom.Element) return null;
       if (grandchild.localName != 'code') return null;