Fix status code and alert structure for sslkeys endpoint, when no ssl keys are present.

[srijeet0406]

This PR fixes #4393.

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Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Follow the steps in the related Github issue #4393.

If this is a bugfix, which Traffic Control versions contained the bug?

• master

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[codecov]

Codecov Report

Merging #7595 (59d406d) into master (c1e70b7) will decrease coverage by 34.51%.
The diff coverage is 0.00%.

@@              Coverage Diff              @@
##             master    #7595       +/-   ##
=============================================
- Coverage     64.80%   30.29%   -34.51%     
  Complexity       98       98               
=============================================
  Files           201      793      +592     
  Lines          8158    83442    +75284     
  Branches        898      898               
=============================================
+ Hits           5287    25281    +19994     
- Misses         2689    56040    +53351     
- Partials        182     2121     +1939     

Flag	Coverage Δ
golib_unit	48.58% <ø> (?)
grove_unit	4.60% <ø> (?)
t3c_unit	5.31% <ø> (?)
traffic_monitor_unit	21.35% <ø> (?)
traffic_ops_integration	69.42% <ø> (-9.22%)	⬇️
traffic_ops_unit	23.17% <0.00%> (?)
traffic_portal_v2	74.25% <ø> (-0.03%)	⬇️
traffic_stats_unit	10.14% <ø> (?)
unit_tests	27.41% <0.00%> (-46.87%)	⬇️
v3	57.79% <ø> (?)
v4	79.18% <ø> (?)
v5	78.63% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...fic_ops/traffic_ops_golang/deliveryservice/keys.go	31.59% <0.00%> (ø)
..._ops/traffic_ops_golang/deliveryservice/sslkeys.go	0.00% <0.00%> (ø)
..._golang/trafficvault/backends/postgres/postgres.go	0.47% <0.00%> (ø)

... and 592 files with indirect coverage changes

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[shamrickus]

LGTM

• Fixes the issue in v5 only
• Returns the correct status code
• TP integration tests pass locally

[zrhoffman]

Reverted in 7d5cebe due to breaking GET /deliveryservices/xmlId/{xmlId}/sslkeys when SSL keys are not present. @srijeet0406 would you please re-PR #7595 now that we know to look out for this regression?