fix(mcphub-nvim): add nil check in system_prompt configuration (#1582)

fix(mcphub): add nil check in system_prompt configuration

Author: zsmatrix62

lua/astrocommunity/editing-support/mcphub-nvim/init.lua

@@ -29,7 +29,7 @@ return {
       opts = {
         system_prompt = function()
           local hub = require("mcphub").get_hub_instance()
-          return hub:get_active_servers_prompt()
+          return hub and hub:get_active_servers_prompt() or ""
         end,
         -- The custom_tools type supports both a list and a function that returns a list. Using a function here prevents requiring mcphub before it's loaded
         custom_tools = function()