Single Subdoc Operation With Short Results Throw NRE Reading Content

Steps to reproduce:

1. Disable enhanced durability
2. Create document with a small integer value
3. Use MutateInBuilder to build a subdocument operation with just one Counter operation against the value
4. Execute the operation
5. Call Content on the operation giving the same subdoc path
6. Receive a null reference exception

Or, for lookups:

1. Enhanced durability is not a factor, fails in both cases
2. Create document with a small integer value
3. Use LookupInBuilder to build a get operation against the value
4. Execute the operation
5. Call Content on the operation giving the same subdoc path
6. Receive a null reference exception

System.NullReferenceException: Object reference not set to an instance of an object.
at Couchbase.DocumentFragment`1.Content[TContent](String path)

This problem doesn't occur if you have more than one operation in the mutation.  The "Bytes" value on fragement value within the DocumentResult<T> is null.