Saving of cached values for grades doesn't match cache miss check

[brianhogg]

Reproduction Steps

1. Install and activate Redis Object Cache plugin (or any persistent object cache)
2. Enroll a student in a course that has a lesson with a quiz assigned
3. Complete the quiz as the student (score does not matter, e.g. 90%)
4. Navigate to the student's grades view (My Grades, Admin Reporting, or Group
   Reporting)
5. Observe: Overall Grade shows 0%, lesson Grade column shows a dash
6. Verify in wp_lifterlms_quiz_attempts that the quiz attempt exists with the correct grade
   and status='pass'

Expected Behavior

Overall Grade displays 90%, lesson Grade column displays 90%.

Actual Behavior

Overall Grade displays 0%, lesson Grade column displays N/A (dash).

Full report on HS-272958

Tech Notes

We are saving the overall grade as '' (empty string) in model.llms.quiz.attempt.php

lifterlms/includes/models/model.llms.quiz.attempt.php

Line 249 in 3a04d99

$this->get_student()->set( 'overall_grade', '' );

and as null if there's no quiz attempt in class-llms-grades.php

lifterlms/includes/class-llms-grades.php

Line 140 in 3a04d99

$grade = null;

but then comparing to false to see if we need to re-calculate

lifterlms/includes/class-llms-grades.php

Lines 219 to 233 in 3a04d99

	$grade = $use_cache ? $this->get_grade_from_cache( $post, $student ) : false;
	// Grade not found in cache or we're not using the cache.
	if ( false === $grade ) {
	$grade = $this->calculate_grade( $post, $student );
	// Store in the cache.
	wp_cache_set(
	sprintf( '%d_grade', $post->get( 'id' ) ),
	$grade,
	sprintf( 'student_%d', $student->get( 'id' ) )
	);
	}

[brianhogg]

There's likely other places we're using null for cached values or places where non-false could be stored.