Fix Internal Server Error when a Delivery Service's orgServerFqdn is an empty string, in some situations

[ocket8888]

Fixes #6775

Adds a check that string pointers which are "required" fields for a given DS type not be empty, whereas previously it only checked if they were nil.

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Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Run the provided unit tests

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

• 6.x
• 5.x

PR submission checklist

• This PR has tests
• This PR doesn't need documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY

[codecov]

Codecov Report

Merging #7382 (d357c0d) into master (b8159ab) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #7382      +/-   ##
============================================
+ Coverage     26.12%   26.15%   +0.03%     
  Complexity       98       98              
============================================
  Files           648      650       +2     
  Lines         76280    76287       +7     
  Branches         90       90              
============================================
+ Hits          19925    19951      +26     
+ Misses        54520    54500      -20     
- Partials       1835     1836       +1     

Flag	Coverage Δ
traffic_ops_integration	69.42% <ø> (+0.07%)	⬆️
v3	57.79% <ø> (ø)
v4	79.18% <ø> (+0.09%)	⬆️
v5	78.63% <ø> (+0.09%)	⬆️

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

Impacted Files	Coverage Δ
...fic_ops_golang/deliveryservice/deliveryservices.go	11.68% <0.00%> (+0.62%)	⬆️

... and 3 files with indirect coverage changes

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