Panic on assign to nil

[gnanderson]

On a new installation of the operator, the following line causes a panic if there are zero annotations in the AmbassadorInstallation object.

https://github.com/datawire/ambassador-operator/blob/febf53e6cf44183301ec9b8b70d687dac3d0deab/pkg/controller/ambassadorinstallation/comp.go#L86