Fix Internal Server Error when GETting sslkey_expirations endpoint

[srijeet0406]

This PR closes #7293

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Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Hit the sslkeys_expirations endpoint and make sure you see the expected result without errors.

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 #7294 (0cb99be) into master (1c5607c) will decrease coverage by 2.18%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #7294      +/-   ##
============================================
- Coverage     28.29%   26.11%   -2.18%     
  Complexity       98       98              
============================================
  Files           617      648      +31     
  Lines         69652    76057    +6405     
  Branches         90       90              
============================================
+ Hits          19705    19862     +157     
- Misses        48127    54362    +6235     
- Partials       1820     1833      +13     

Flag	Coverage Δ
golib_unit	53.02% <ø> (-0.08%)	⬇️
grove_unit	4.60% <ø> (ø)
t3c_unit	5.32% <ø> (?)
traffic_monitor_unit	20.43% <ø> (ø)
traffic_ops_integration	69.34% <ø> (+0.05%)	⬆️
traffic_ops_unit	19.67% <0.00%> (-0.01%)	⬇️
traffic_stats_unit	10.41% <ø> (ø)
unit_tests	22.54% <0.00%> (?)
v3	57.79% <ø> (+0.11%)	⬆️
v4	79.09% <ø> (ø)
v5	78.53% <ø> (ø)

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

Impacted Files	Coverage Δ
..._golang/trafficvault/backends/postgres/postgres.go	0.47% <0.00%> (ø)
lib/go-tc/deliveryservice_requests.go	40.86% <0.00%> (ø)
...fic_ops_golang/deliveryservice/request/requests.go	0.00% <0.00%> (ø)
...fic_ops_golang/deliveryservice/request/validate.go	0.00% <0.00%> (ø)
lib/go-tc/copy.go
cache-config/t3c-apply/util/gitutil.go	0.00% <0.00%> (ø)
cache-config/t3c/t3c.go	0.00% <0.00%> (ø)
cache-config/t3cutil/actionlog.go	0.00% <0.00%> (ø)
cache-config/t3cutil/toreq/clientfuncs.go	0.00% <0.00%> (ø)
cache-config/t3c-tail/t3c-tail.go	0.00% <0.00%> (ø)
... and 31 more

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[ocket8888]

Other than the Changelog, this looks good to go. Tested it and it's working.