fix(iaas): safely check resp.NetworkInterface for nil (#1497)

NullableString.IsSet() returns true even if the value is nil:

```
func (v *NullableString) UnmarshalJSON(src []byte) error {
	v.isSet = true
	return json.Unmarshal(src, &v.value)
}
```

Author: cgoetz-inovex

internal/pkg/services/iaas/utils/utils.go

@@ -39,8 +39,8 @@ func GetPublicIP(ctx context.Context, apiClient iaas.DefaultAPI, projectId, regi
 		return "", "", fmt.Errorf("get public ip: %w", err)
 	}
 	associatedResourceId := ""
-	if resp.NetworkInterface.IsSet() {
-		associatedResourceId = *resp.NetworkInterface.Get()
+	if networkInterface := resp.NetworkInterface.Get(); networkInterface != nil {
+		associatedResourceId = *networkInterface
 	}
 	return *resp.Ip, associatedResourceId, nil
 }

internal/pkg/services/iaas/utils/utils_test.go

@@ -263,6 +263,17 @@ func TestGetPublicIp(t *testing.T) {
 			},
 			wantErr: true,
 		},
+		{
+			name: "nil network interface",
+			args: args{
+				getPublicIpResp: &iaas.PublicIp{
+					Ip:               utils.Ptr("1.2.3.4"),
+					NetworkInterface: *iaas.NewNullableString(nil),
+				},
+			},
+			wantPublicIp:           "1.2.3.4",
+			wantAssociatedResource: "",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {