Fix issue #1006, remove nil type from markdown lsp server.

README.md

@@ -264,7 +264,7 @@ lsp-bridge provides support for more than two language servers for many language
 - `lsp-bridge-csharp-lsp-server`: C# language server, you can choose `omnisharp-mono`, `omnisharp-dotnet` or `csharp-ls`, note that you need to give **execute permissions** to the OmniSharp file
 - `lsp-bridge-python-multi-lsp-server`: Python multi-language servers, you can choose `basedpyright_ruff`, `pyright_ruff`, `jedi_ruff`, `python-ms_ruff`, `pylsp_ruff`
 - `lsp-bridge-nix-lsp-server`: Nix language server, you can choose `rnix-lsp`, `nixd` or `nil`
-- `lsp-bridge-markdown-lsp-server`: Markdown language server, you can choose `vale-ls` `nil` or `marksman` 
+- `lsp-bridge-markdown-lsp-server`: Markdown language server, you can choose `vale-ls` or `marksman` 
 - `lsp-bridge-lua-lsp-server`: Lua language server, you can choose `sumneko` or `lua-lsp` 
 
 ## Options

README.zh-CN.md

@@ -263,7 +263,7 @@ lsp-bridge 针对许多语言都提供 2 个以上的语言服务器支持， 
 - `lsp-bridge-csharp-lsp-server`: C#语言的服务器， 可以选择`omnisharp-mono`, `omnisharp-dotnet` 或者 `csharp-ls`, 注意你需要给 OmniSharp 文件**执行权限**才能正常工作
 - `lsp-bridge-python-multi-lsp-server`: Python 多语言服务器， 可以选择 `basedpyright_ruff`, `pyright_ruff`, `jedi_ruff`, `python-ms_ruff`, `pylsp_ruff`
 - `lsp-bridge-nix-lsp-server`: Nix 语言的服务器， 可以选择 `rnix-lsp`, `nixd` 或者 `nil`
-- `lsp-bridge-markdown-lsp-server`: Markdown 语言的服务器， 可以选择 `vale-ls`, `nil` 或者 `marksman`
+- `lsp-bridge-markdown-lsp-server`: Markdown 语言的服务器， 可以选择 `vale-ls` 或者 `marksman`
 - `lsp-bridge-lua-lsp-server`: Lua 语言的服务器， 可以选择 `sumneko`, 或者 `lua-lsp`
 
 ## 选项

lsp-bridge.el

@@ -501,8 +501,8 @@ Possible choices are basedpyright_ruff, pyright_ruff, pyright-background-analysi
   "Default LSP server for nix, you can choose `rnix-lsp', `nixd' or `nil'."
   :type 'string)
 
-(defcustom lsp-bridge-markdown-lsp-server "nil"
-  "Default LSP server for markdown, you can choose `vale-ls', `nil' or `marksman'."
+(defcustom lsp-bridge-markdown-lsp-server "vale-ls"
+  "Default LSP server for markdown, you can choose `vale-ls' or `marksman'."
   :type 'string)
 
 (defcustom lsp-bridge-lua-lsp-server "sumneko"