fix: prevent extra query and invalid size in Model::chunk()

[michalsn]

Description
This PR fixes a bug where Model::chunk() ran an unnecessary extra database query at the end of iteration.

Additionally, chunk() now throws InvalidArgumentException when called with a chunk size of zero or less. Previously, passing 0 as the size would silently interact with the Config\Feature::limitZeroAsAll feature flag, potentially converting LIMIT 0 into an unlimited query and loading the entire table into memory at once.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide