[dns|aws] Return empty set on route53 if no records match on mock

On route53 AWS dns provider, when getting records by type a exception is thrown if no record matches. This behavior is different as the real one, that returns *nil* if no record matches instead of raising an exception.

This patch handles that case and creates an empty array in this case.

```ruby
require 'fog/aws/dns'
Fog.mock!
aws = { 
  :provider => 'AWS',
  :aws_access_key_id => 'KEY_ID',
  :aws_secret_access_key => 'SECRET_KEY'
} 


zone = Fog::DNS.new(aws).zones.create(:domain => 'wadus')
zone.records.get('wadus', 'AAAA')

NoMethodError: undefined method `values' for nil:NilClass
from gems/fog-1.19.0/lib/fog/aws/requests/dns/list_resource_record_sets.rb:79:in `list_resource_record_sets'

Real behaviour:

```
zone.records.get('wadus', 'AAAA')                                                                                               
=> nil
```

Patched:

````
zone.records.get('wadus', 'AAAA')
=> nil
```