MdeModulePkg/FvSimpleFileSystem: fix assertions when FV is empty

The original code will assert when dealing with those empty FVs.
The fix is used to solve this bug.

Cc: Chao Zhang <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>

MdeModulePkg/Universal/FvSimpleFileSystemDxe/FvSimpleFileSystemEntryPoint.c

@@ -223,7 +223,11 @@ FvSimpleFileSystemOpenVolume (
     }
   }
 
-  Instance->Root->DirReadNext = FVFS_GET_FIRST_FILE_INFO (Instance);
+  Instance->Root->DirReadNext = NULL;
+  if (!IsListEmpty (&Instance->FileInfoHead)) {
+    Instance->Root->DirReadNext = FVFS_GET_FIRST_FILE_INFO (Instance);
+  }
+
   *RootFile = &Instance->Root->FileProtocol;
   return Status;
 }