Check if we have a return from mmcli -L before using it in Wizard

luci-app-openmptcprouter/luasrc/view/openmptcprouter/wizard.htm

@@ -46,6 +46,7 @@
 
 	alltty = {}
 	iftty = luci.sys.exec("timeout 1 /usr/bin/mmcli -L")
+	if iftty ~= nil then
 		for listtty in iftty:gmatch("([^\r\n]*)[\r\n]") do
 			modemid = luci.util.trim(luci.sys.exec("echo '" .. listtty .. "' | awk -F' ' '{print $1}' | awk -F/ '{print $6}'"))
 			if modemid ~= '' then
@@ -54,6 +55,7 @@
 				table.insert(alltty, tty)
 			end
 		end
+	end
 
 
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