Deleting a binding on a non-existing RabbitMQ Cluster causes a panic

[Zerpet]

Describe the bug

When a Binding object is marked for deletion and RabbitMQ Cluster does not exist, the operator panics.

To Reproduce

Steps to reproduce the behavior: Untested

1. Create a RabbitMQ cluster and a valid binding
2. Delete RabbitMQ cluster
3. Delete the binding

Include any YAML or manifest necessary to reproduce the problem.

Expected behavior
The Operator deletes the binding object and/or removes the finalizer in the Binding object.

Screenshots

Log snippet

2021-04-06T09:07:59.848Z        INFO    controller-runtime.manager.controller.binding   Deleting        {"reconciler group": "rabbitmq.com", "reconciler kind": "Binding", "name": "x-invoices-to-invoices", "namespace": "rabbitmq-red"}
E0406 09:07:59.849198       1 runtime.go:78] Observed a panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)
goroutine 374 [running]:
k8s.io/apimachinery/pkg/util/runtime.logPanic(0x1581480, 0x22be540)
        /go/pkg/mod/k8s.io/[email protected]/pkg/util/runtime/runtime.go:74 +0x95
k8s.io/apimachinery/pkg/util/runtime.HandleCrash(0x0, 0x0, 0x0)
        /go/pkg/mod/k8s.io/[email protected]/pkg/util/runtime/runtime.go:48 +0x86
panic(0x1581480, 0x22be540)
        /usr/local/go/src/runtime/panic.go:965 +0x1b9
github.com/michaelklishin/rabbit-hole/v2.newRequestWithBody(0x0, 0x171d9c8, 0x6, 0xc00099e990, 0x26, 0x0, 0x0, 0x0, 0x7f9f229665a0, 0xc0005e9728, ...)
        /go/pkg/mod/github.com/michaelklishin/rabbit-hole/[email protected]/client.go:100 +0x37
github.com/michaelklishin/rabbit-hole/v2.(*Client).DeleteBinding(0x0, 0x22808b8, 0x1, 0xc000047d30, 0xa, 0x22808b8, 0x1, 0xc000047d20, 0x8, 0xc000047d28, ...)
        /go/pkg/mod/github.com/michaelklishin/rabbit-hole/[email protected]/bindings.go:217 +0x330
github.com/rabbitmq/messaging-topology-operator/controllers.(*BindingReconciler).deleteBinding(0xc000422900, 0x197ab58, 0xc00077d590, 0x0, 0xc000754d00, 0x0, 0xc000047cd0)
        /workspace/controllers/binding_controller.go:156 +0x17b
github.com/rabbitmq/messaging-topology-operator/controllers.(*BindingReconciler).Reconcile(0xc000422900, 0x197ab58, 0xc00077d590, 0xc000047cd0, 0xc, 0xc000403170, 0x16, 0xc00077d590, 0xc000030000, 0x15feb80, ...)
        /workspace/controllers/binding_controller.go:67 +0xcfe
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).reconcileHandler(0xc0002e8140, 0x197aab0, 0xc000326000, 0x15cb320, 0xc000774a80)
        /go/pkg/mod/sigs.k8s.io/[email protected]/pkg/internal/controller/controller.go:298 +0x30d

Additional context

The problem seems to be in Reconcile() function, which does not cover the case of a binding marked for deletion and RabbitMQ Cluster object does not exist.

messaging-topology-operator/controllers/binding_controller.go

Lines 59 to 65 in 02fc8f0

	if errors.Is(err, internal.NoSuchRabbitmqClusterError) && binding.ObjectMeta.DeletionTimestamp.IsZero() {
	logger.Info("Could not generate rabbitClient for non existent cluster: " + err.Error())
	return reconcile.Result{RequeueAfter: 10 * time.Second}, err
	} else if err != nil && !errors.Is(err, internal.NoSuchRabbitmqClusterError) {
	logger.Error(err, failedGenerateRabbitClient)
	return reconcile.Result{}, err
	}

We have done a quick spike to verify this, but we should fix this properly and add a test using the framework in #89.