fix: allow empty security requirements

[tofran]

Sorry this was supposed to be created in draft mode. Will remove this message and make it ready to review when pending topics are resolved.

Closes #4016

Made the implementation of resolve_security is a little naive and readable, see if you like it.

For discussion:

If I have an App or Router with security and for a specific route, set an empty security, should it override the global policy? This would mean dropping the security from the previous layer.

What if the route wants to add a new one instead of overriding? I think these topics need to be addressed before closing the issue.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.35%. Comparing base (6768845) to head (cf3be16).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4018   +/-   ##
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  Coverage   98.35%   98.35%           
=======================================
  Files         347      347           
  Lines       15783    15786    +3     
  Branches     1745     1746    +1     
=======================================
+ Hits        15523    15526    +3     
  Misses        124      124           
  Partials      136      136           

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[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/4018