properly detect LSP client after LSP restart (closes issue #174)

lua/fzf-lua/providers/lsp.lua

@@ -395,7 +395,7 @@ M.diagnostics = function(opts)
   if not opts then return end
 
   local lsp_clients = vim.lsp.buf_get_clients(0)
-  if #lsp_clients == 0 then
+  if utils.tbl_length(lsp_clients) == 0 then
     utils.info("LSP: no client attached")
     return
   end
@@ -568,7 +568,7 @@ local function check_capabilities(feature)
   if supported_client then
     return true
   else
-    if #clients == 0 then
+    if utils.tbl_length(clients) == 0 then
       utils.info("LSP: no client attached")
     else
       utils.info("LSP: server does not support " .. feature)