Fix TO cdns/dnsseckeys to default if no ttl exists

[rob05c]

Fixes #3204

API Tests don't support Riak yet.
Manually Tested against a TO environment with Riak, with DNSSEC keys. Deleted the tld.ttls.DS and Snapshotted the CDN, verified an Internal Server Error without this fix, endpoint works as expected and logs errors with the fix.

What does this PR do?

Fixes #3204

Which TC components are affected by this PR?

• Documentation
• Grove
• Traffic Analytics
• Traffic Monitor
• Traffic Ops
• Traffic Ops ORT
• Traffic Portal
• Traffic Router
• Traffic Stats
• Traffic Vault
• Other _________

What is the best way to verify this PR?

On a TO deployment with DNSSEC and Riak, delete the tld.ttls.DS Parameter and Snapshot the CRConfig for the CDN. Without this PR, /api/1.4/cdns/name/{cdn-name}/dnsseckeys will return an Internal Server Error, with this PR, endpoint will succeed.

Check all that apply

• This PR includes tests
• This PR includes documentation updates
• This PR includes an update to CHANGELOG.md
• This PR includes all required license headers
• This PR includes a database migration (ensure that migration sequence is correct)
• This PR fixes a serious security flaw. Read more: www.apache.org/security

[asfgit]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/trafficcontrol-PR/3037/
Test PASSed.