修复无法新增公告

src/Hotline.Application/Snapshot/SnapshotBulletinApplication.cs

@@ -187,6 +187,8 @@ public class SnapshotBulletinApplication : ISnapshotBulletinApplication, IScopeD
         var model = dto.Adapt<SnapshotBulletin>();
         model.BulletinState = EBulletinState.Draft;
         model.ReadedNum = 0;
+        if (model.Content.IsNullOrEmpty())
+            model.Content = " ";
         if (model.BulletinTime.HasValue == false) model.BulletinTime = DateTime.Now;
         return await _bulletinRepository.AddAsync(model, token);
     }