CP-17382 skipattributes no longer skips mysql

[niden]

Hello!

• Type: bug fix
• Link to issue: [BUG]:Model::skipAttributes() no longer skips a MySQL generated column during INSERT #17382

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• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist
• I wrote some tests for this PR
• I have updated the relevant CHANGELOG
• I have created a PR for the documentation about this change

Fixed Phalcon\Mvc\Model ignoring attributes registered with skipAttributes(), skipAttributesOnCreate() and skipAttributesOnUpdate(), so a skipped column was emitted in the generated INSERT/UPDATE (breaking, for instance, inserts into a table with a MySQL generated column). The skip list is keyed with null values, and isset() on an array offset now follows PHP semantics (key present and value not null), which made every skipped attribute read as not registered; the checks in doLowInsert(), doLowUpdate() and the not-null validation now use array_key_exists().

Thanks