Lesson's numerical answers with value 0 are deleted on edit

When editing a numerical question in a lesson, all the answers equal to zero are being deleted.

This has been fixed when adding such an answer (MDL-57366) but editing is still broken.

The field text value is saved into a lesson_page_answer (extending lesson_base) object. And within that class, the isset function is overridden and is returning the empty function result.

Which makes this test https://github.com/moodle/moodle/blob/MOODLE_36_STABLE/mod/lesson/locallib.php#L4479 fail. The answer is therefore dropped.

A simple fix would be to replace the test mentioned above:

 if (isset($this->answers[$i]->answer) && $this->answers[$i]->answer != '') {

with

if ($this->answers[$i]->answer !== null && $this->answers[$i]->answer !== '') {

to avoid calling the "magic" isset function.

In order to reproduce the issue:

• Create a new lesson
• Add a new numerical page
• Add an answer with value of 0
• Save the question
• Edit that question and save it again

=> The answer is dropped.