Check for existence of NEXTAUTH_URL before overwriting

[krumware]

What does this PR do?

This allows NEXTAUTH_URL to be configurable to a different domain than NEXT_PUBLIC_WEBAPP_URL

Fixes # (issue)

closes #5418

Environment: Staging(main branch) / Production

Type of change

• Bug fix (non-breaking change which fixes an issue)

How should this be tested?

• run project at http://tester.localhost:3000 Configure NEXTAUTH_URL as http://anythingthatwillfail:3000
  • login should fail
• test as indicated in (next-auth) Service unable to call back to itself if address doesn't match public URL  #5418
  • login should succeed

Checklist

• I haven't read the contributing guide
• My code doesn't follow the style guidelines of this project
• I haven't performed a self-review of my own code and corrected any misspellings
• I haven't commented my code, particularly in hard-to-understand areas
• I haven't checked if my PR needs changes to the documentation
• I haven't checked if my changes generate no new warnings
• I haven't added tests that prove my fix is effective or that my feature works
• I haven't checked if new and existing unit tests pass locally with my changes

[vercel]

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[what-the-diff]

• The if statement was changed to an if/else
• If the NEXTAUTH_URL is not set, then it will use NEXT_PUBLIC_WEBAPP_URL + "/api/auth" as a fallback

[krumware]

@baileypumfleet