Fix nil interface{} checks

[tpantelis]

PR #1447 changed places where we check for nil interface{} in the callback functions for AwaitUntil to cast first. However this can result in an invalid type conversion panic depending on how the value is returned from the operation function. In Go, an interface var contains both a type and value so there are 2 cases for a nil:

1. The type is non-nil and the value is nil
2. The type and value are nil

For 1, the interface{} var is not actually nil so checking if result == nil yields false. Eg,

  var s *string
  var i interface{} = s

  if i == nil { // yields false b/c i is typed
  }

In this case, we can cast to the expected type and then check for nil.

s := i.(*string)
if s == nil {  // yields true
}

For 2, the interface{} var is untyped so nil check yields true:

var i interface{} = nil
 if i == nil { // yields true
 }

So in cases where we directly return nil from an operation function, the nil check in the result function is valid.

The actual reason for #1447 was b/c of a nil pointer panic in AwaitGatewaysWithStatus. This function was previously changed to call findGateway and return its values. However findGateway returns *unstructured.Unstructured so now the operation function returned a typed interface{} var. So if findGateway returns a nil *unstructured.Unstructured value, the result == nil check in the result function yields false. This resulted in the panic when result was later casted and accessed.

So we basically need to undo #1447. For AwaitGatewayWithStatus and AwaitGatewayFullyConnected, check the error from findGateway and return a nil interface{} value.

[submariner-bot]

🤖 Created branch: z_pr1459/tpantelis/fix_nil_checks

[tpantelis]

The globalnet w/ helm job is failing:

E1106 16:18:52.373476 1 leaderelection.go:327] error retrieving resource lock submariner-operator/submariner-globalnet-lock: leases.coordination.k8s.io "submariner-globalnet-lock" is forbidden: User "system:serviceaccount:submariner-operator:submariner-globalnet" cannot get resource "leases" in API group "coordination.k8s.io" in the namespace "submariner-operator"

Need to update the helm chart to add submariner-globalnet permissions. This issue is orthogonal so shouldn't block this PR so force merging.

[submariner-bot]

🤖 Closed branches: [z_pr1459/tpantelis/fix_nil_checks]