Set collection to None in item constructor when there is no collection

[gadomski]

Related Issue(s):

• Reported in Browseable validation throws KeyError exception stac-api-validator#527

Description:

collection is listed as a top-level attribute for Item, but it is not set when there is no collection, leading to an attribute error when trying to access Item::collection.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.20%. Comparing base (d4bed27) to head (22a78dd).
Report is 34 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1400   +/-   ##
=======================================
  Coverage   91.19%   91.20%           
=======================================
  Files          52       52           
  Lines        7238     7239    +1     
  Branches     1019     1019           
=======================================
+ Hits         6601     6602    +1     
  Misses        457      457           
  Partials      180      180           

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[jsignell]

LGTM