Standardize NULL initialization to be consistent.

usr.sbin/bsdconfig/networking/devices

@@ -71,7 +71,7 @@ f_mustberoot_init
 #
 # Launch application main menu
 #
-defaultitem=""
+defaultitem=
 while :; do
 	f_dialog_menu_netdev "$defaultitem" || break
 	f_dialog_menutag_fetch interface

usr.sbin/bsdconfig/share/media/options.subr

@@ -69,7 +69,7 @@ f_media_options_menu()
 
 	local cp
 	while :; do
-		menu_list=""
+		menu_list=
 
 		f_getvar $VAR_NFS_SECURE cp
 		if [ "$cp" = "YES" ]; then menu_list="$menu_list

usr.sbin/bsdconfig/share/media/tcpip.subr

@@ -1371,7 +1371,7 @@ f_device_dialog_tcp()
 			# Create a persistent menu instead
 
 			f_dialog_title "$msg_network_configuration"
-			local prompt=""
+			local prompt=
 
 			while :; do
 				cp=$( $DIALOG \