V2: handle error when creating a listener with no site

[nluaces]

Describe the bug After creating a listener resource without creating a site, the controller log show this error:

2024/07/03 14:15:32 Waiting for informer caches to sync
2024/07/03 14:15:32 checking listener noe in skupper
2024/07/03 14:15:32 updating listener skupper/noe...
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x158 pc=0x1377eba]

goroutine 1 [running]:
github.com/skupperproject/skupper/pkg/kube/site.(*Site).groups(...)
	/go/src/app/pkg/kube/site/site.go:164
github.com/skupperproject/skupper/pkg/kube/site.(*Site).updateRouterConfigForGroups(0xc00012c000?, {0x1a26e40?, 0xc0005b2500?})
	/go/src/app/pkg/kube/site/site.go:557 +0x3a
github.com/skupperproject/skupper/pkg/kube/site.(*Site).CheckListener(0xc0004aeaa0, {0xc00081636c?, 0xc00077de50?}, 0x7?)
	/go/src/app/pkg/kube/site/site.go:642 +0x4d
main.(*Controller).Run(0xc00034e580, 0xc00010e180)
	/go/src/app/cmd/controller/controller.go:167 +0xb9a

How To Reproduce Steps to reproduce the behavior:

• deploy skupper
• create a listener in the namespace

Expected behavior

• The error is handled and logged
• The listener shows a specific status error

Version 2.X