fix(lsp): vim.lsp.start fails if existing client has no workspace_folders #31608

Problem:
regression since https://github.com/neovim/neovim/pull/31340

`nvim -l repro.lua`:
```lua
vim.lsp.start { cmd = { 'lua-language-server' }, name = 'lua_ls' }
vim.lsp.start { cmd = { 'lua-language-server' }, name = 'lua_ls', root_dir = 'foo' }

-- swapped case will be ok:
-- vim.lsp.start { cmd = { 'lua-language-server' }, name = 'lua_ls', root_dir = 'foo' }
-- vim.lsp.start { cmd = { 'lua-language-server' }, name = 'lua_ls' }
```

Failure:
```
E5113: Error while calling lua chunk: /…/lua/vim/lsp.lua:214: bad argument #1 to
'ipairs' (table expected, got nil)
stack traceback:
        [C]: in function 'ipairs'
        /…/lua/vim/lsp.lua:214: in function 'reuse_client'
        /…/lua/vim/lsp.lua:629: in function 'start'
        repro.lua:34: in main chunk
```

runtime/lua/vim/lsp.lua

@@ -211,7 +211,7 @@ local function reuse_client_default(client, config)
 
   for _, config_folder in ipairs(config_folders) do
     local found = false
-    for _, client_folder in ipairs(client.workspace_folders) do
+    for _, client_folder in ipairs(client.workspace_folders or {}) do
       if config_folder.uri == client_folder.uri then
         found = true
         break

test/functional/plugin/lsp_spec.lua

@@ -1854,6 +1854,20 @@ describe('LSP', function()
         end,
       }
     end)
+
+    it('vim.lsp.start when existing client has no workspace_folders', function()
+      exec_lua(create_server_definition)
+      eq(
+        { 2, 'foo', 'foo' },
+        exec_lua(function()
+          local server = _G._create_server()
+          vim.lsp.start { cmd = server.cmd, name = 'foo' }
+          vim.lsp.start { cmd = server.cmd, name = 'foo', root_dir = 'bar' }
+          local foos = vim.lsp.get_clients()
+          return { #foos, foos[1].name, foos[2].name }
+        end)
+      )
+    end)
   end)
 
   describe('parsing tests', function()