protocols: make APN option optional again

Fixes: #5284
Fixes: 1661ac6b ("luci-proto-ncm: add APN and PIN validation")
Fixes: 114dc38d ("luci-proto-qmi: add APN and PIN validation")
Fixes: 6a8ad40b ("luci-proto-3g: add APN and PIN validation")
Fixes: ebb676ef

 ("luci-proto-modemmanager: add APN and PIN validation")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>