The custom report exporter defines all data from additional exporters as non-optional: https://github.com/moodle/moodle/blob/941a29925e5cd1c1dddefa08cede7950593080b0/reportbuilder/classes/external/custom_report_exporter.php#L92-L96

This means all additional exporters are defining each of their own properties as optional: https://github.com/moodle/moodle/blob/941a29925e5cd1c1dddefa08cede7950593080b0/reportbuilder/classes/external/custom_report_filters_exporter.php#L54-L96 - despite always populating them

The properties on the main custom report exporter should be optional (to define whether they exist at all), the properties on the additional exporters should only be optional when necessary

Noticed while writing unit tests in MDL-74694. Moved to this issue as probably master only change, as will slightly amend the return structure of various external methods that use the read definition of exporters