Fix errors deleting Route 53 records

[rossf7]

I hit an error deleting Route 53 records in my local testing that leaves dangling records.

/.gvm/pkgsets/go1.8/giantswarm/src/github.com/giantswarm/aws-operator/resources/aws/record_set.go:43: } {/Users/ross/.gvm/pkgsets/go1.8/giantswarm/src/github.com/giantswarm/aws-operator/resources/aws/record_set.go:57: } {InvalidInput: Invalid XML ; cvc-enumeration-valid: Value '' is not facet-valid with respect to enumeration '[SOA, A, TXT, NS, CNAME, MX, NAPTR, PTR, SRV, SPF, AAAA, CAA]'. It must be a value from the enumeration.\n\tstatus code: 400, request id: 609fb88c-7211-11e7-9d3b-997b1216adfb}]","time":"17-07-26 14:47:43.853"}

Lorenzo already did a fix for this in #315