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|