[DNS] Ensure that ManagedZone:exists() does not misreport True result.

[charles-salmon]

The behaviour of the MangedZones: get endpoint is such that when only the id field of a non-existent ManagedZone is requested, a 200 status code response with body {} is returned.

This is arguably undesirable behaviour, however this patch serves to ensure that the Python SDK behaves expectedly with the API in its current state.

Is anyone aware of where the best place to leave feedback on the API would be? Is the "Leave Feedback" section on the documentation page appropriate? Is it an open-source project?

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[charles-salmon]

My employer signed the corporate CLA, and added me to the authorized contributors group, approximately an hour ago.

[tseaver]

@charles-salmon Thank you very much for digging into this one!

Closes #5818.

[tseaver]

@charles-salmon Can you reply with literally the text "I signed it!" so the CLA bot will check again?

[charles-salmon]

I signed it!

[charles-salmon]

(I think the corporate CLA is pending approval.)

[tseaver]

@theacodes, @JustinBeckwith Can one of you figure out what might be blocking the CLA bot here?

[charles-salmon]

I signed it!

[googlebot]

CLAs look good, thanks!

[charles-salmon]

(My employer notified me that the CLA was approved.)