Fix: Quiz answer image_id empty string fails on MariaDB strict mode

[shsajalchowdhury]

Summary

When saving a quiz answer without an image, the image_id value was passed as an empty string ('') to wpdb->insert(). On MariaDB strict mode (and MySQL strict mode), this causes a database insertion failure because the image_id column is defined as bigint(20) DEFAULT NULL — it only accepts integers or NULL, not empty strings.

Root Cause

1. QuizBuilder::prepare_answer_data() calls Input::sanitize($input['image_id'] ?? null) which returns an empty string '' when no image is provided
2. wpdb->insert() receives the empty string and passes it through wpdb->prepare('%s', '') producing '' in the SQL
3. The image_id column (bigint(20) DEFAULT NULL) rejects '' on strict SQL modes

Changes Made

File: classes/QuizBuilder.php

1. prepare_answer_data() (line 123) — After sanitization, convert empty image_id to null:

   $image_id = empty( $image_id ) ? null : (int) $image_id;

2. save_question_answers() (line 177) — Filter out null values before wpdb->insert() so MySQL uses the column DEFAULT NULL:

   $insert_data = array_filter(
       $answer_data,
       function ( $value ) { return null !== $value; }
   );
   $wpdb->insert( $answers_table, $insert_data );

Steps to Reproduce

1. Use MariaDB with strict mode (sql_mode = STRICT_TRANS_TABLES)
2. Create a quiz with any question type that has answer options
3. Add an answer without selecting an image
4. Save the quiz question
5. Before fix: Answer fails to save (database error in debug log)
6. After fix: Answer saves correctly with image_id = NULL

Testing Instructions

1. Switch DB to MariaDB with STRICT_TRANS_TABLES mode
2. Create a quiz, add a question, add answers without images
3. Verify answers are saved successfully
4. Check the tutor_quiz_question_answers table — image_id should be NULL
5. Also test with MySQL non-strict mode to verify no regression

Does this PR change what data or activity we track or use?

No.

Fixes #1894