fix(lsp): do not assume client capability exists in watchfiles check (#24558)

Backports #24550.

Adjusts test to expect that didChangeWatchedFiles is *not* registered if
client `capabilities` is nil (as it's not enabled by default for v0.9).

runtime/lua/vim/lsp.lua

@@ -933,8 +933,7 @@ end
 ---       |vim.lsp.protocol.make_client_capabilities()|, passed to the language
 ---       server on initialization. Hint: use make_client_capabilities() and modify
 ---       its result.
----       - Note: To send an empty dictionary use
----         `{[vim.type_idx]=vim.types.dictionary}`, else it will be encoded as an
+---       - Note: To send an empty dictionary use |vim.empty_dict()|, else it will be encoded as an
 ---         array.
 ---
 --- - handlers: Map of language server method names to |lsp-handler|

runtime/lua/vim/lsp/_watchfiles.lua

@@ -193,12 +193,15 @@ local to_lsp_change_type = {
 function M.register(reg, ctx)
   local client_id = ctx.client_id
   local client = vim.lsp.get_client_by_id(client_id)
-  if
-    -- Ill-behaved servers may not honor the client capability and try to register
-    -- anyway, so ignore requests when the user has opted out of the feature.
-    not client.config.capabilities.workspace.didChangeWatchedFiles.dynamicRegistration
-    or not client.workspace_folders
-  then
+  -- Ill-behaved servers may not honor the client capability and try to register
+  -- anyway, so ignore requests when the user has opted out of the feature.
+  local has_capability = vim.tbl_get(
+    client.config.capabilities or {},
+    'workspace',
+    'didChangeWatchedFiles',
+    'dynamicRegistration'
+  )
+  if not has_capability or not client.workspace_folders then
     return
   end
   local watch_regs = {}