Fix broken populate traversal with no fragment

[Convly]

What does it do?

Actual fix here, the rest of the changes are tests added to check we don't have this regression in the future

Prevent recursion on undefined populate fragment values upon populate traversal (during query sanitization)

Why is it needed?

Null & undefined values for the populate fragment upon query traversal cause the traversal to throw due to an unexpected Object.entries operation on an undefined value.

How to test it?

Using the API integration tests:
yarn test:api packages/core/strapi/tests/api/sanitize/sanitize-query.test.api.js

Or by starting the getstarted app, creating a new restaurant entity from the content manager, then heading to the content API and trying to access this URL
http://localhost:1337/api/restaurants?populate[dz][populate]=*
It should return you the restaurant and the content of its dynamic zone populated (at the second level)

Related issue(s)/PR(s)

fix #16105

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@a9d1afd). Click here to learn what that means.
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16109   +/-   ##
=======================================
  Coverage        ?   59.95%           
=======================================
  Files           ?     1484           
  Lines           ?    37346           
  Branches        ?     7488           
=======================================
  Hits            ?    22389           
  Misses          ?    12769           
  Partials        ?     2188           

Flag	Coverage Δ
back	50.06% <0.00%> (?)
front	66.21% <ø> (?)
unit_back	50.06% <0.00%> (?)
unit_front	66.21% <ø> (?)

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

Impacted Files	Coverage Δ
packages/core/utils/lib/traverse/query-populate.js	9.70% <0.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[innerdvations]

LGTM!

• local tests pass and also tested manually