Add nil check on Document Render to avoid panic

[inFocus7]

• Expanded the existing check during Render to generally check for a nil highOpenAPI3Model, with more specific error message depending on if it's Swagger or not.
• Also added a check for the info which is a pointer, so could technically be nil.
• Added a test case to ensure we error. Without my changes the test panics which we're trying to avoid.

Context: I've previously hit a panic while learning about & implementing the library. It took me a while to figure out why and got me a bit spooked to have this in a long-running cluster/container. I added a recover in my end, but handling it here cleanly is nicer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.62%. Comparing base (5ac8657) to head (19298a8).
Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #343   +/-   ##
=======================================
  Coverage   99.62%   99.62%           
=======================================
  Files         165      165           
  Lines       21050    21056    +6     
=======================================
+ Hits        20971    20977    +6     
  Misses         74       74           
  Partials        5        5           

Flag	Coverage Δ
unittests	99.62% <100.00%> (+<0.01%)	⬆️

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[daveshanley]

LGTM! Thank you for your contribution.