Taken from: http://www.uni-kiel.de/unizeit/index.php?bid=630202
»Haste nichts, dann machste halt was«, darf dagegen der norddeutsche Internet-User schriftlich unwidersprochen behaupten.
[emphasis added]
I suppose it's a regional or colloquial expression. It seems to mean
"mach etwas!"
as in "simply/just do something!"
Is my understanding correct?