Ubuntu server 16.04 mdadm.conf defines no arrays

Whilst performing updates on Ubuntu server 16.04 we see the following warning. (Accepting it is a warning, it is none-the-less annoying).

Setting up initramfs-tools (0.122ubuntu8.3) ...
update-initramfs: deferring update (trigger activated)
Processing triggers for initramfs-tools (0.122ubuntu8.3) ...
update-initramfs: Generating /boot/initrd.img-4.4.0-38-generic
W: mdadm: /etc/mdadm/mdadm.conf defines no arrays.

The warning is displayed whenever there’s no ARRAY line in mdadm.conf

The simplest fix for this warning is to add the following line to the end of /etc/mdadm/mdadm.conf

ARRAY <ignore> devices=/dev/sda