fix: check if there is cmd input before attempting to add to history (closes #206)

nature/opts/history.lua

@@ -1,5 +1,6 @@
 local bait = require 'bait'
 
 bait.catch('command.exit', function(_, cmd, priv)
+	if not cmd then return end
 	if not priv and hilbish.opts.history then hilbish.history.add(cmd) end
 end)