Update hangup_rx.lua

Prevent error if the value of x is nil.

app/switch/resources/scripts/app/fax/resources/scripts/hangup_rx.lua

@@ -63,6 +63,7 @@
 
 --escape shell arguments to prevent command injection
 	local function shell_esc(x)
+		if (x == nil) then return '' end
 		return (x:gsub('\\', '\\\\')
 			:gsub('\'', '\\\''))
 	end