fix(lsp): don't use completion filterText if prefix is empty

Follow up to https://github.com/neovim/neovim/pull/32072

If there is no prefix (e.g. at the start of word boundary or a line), it
always used the `filterText` because the `match` function always
returned false.

runtime/lua/vim/lsp/completion.lua

@@ -231,6 +231,9 @@ end
 ---@param prefix string
 ---@return boolean
 local function match_item_by_value(value, prefix)
+  if prefix == '' then
+    return true
+  end
   if vim.o.completeopt:find('fuzzy') ~= nil then
     return next(vim.fn.matchfuzzy({ value }, prefix)) ~= nil
   end