[#143][#144] Change !is_null to isset

[owenherbert-catalyst]

This merge request fixes issues (#143 & #144) where accessing $fromform->saveandnext, $fromform->abandonbutton, $fromform->submitobservation could trigger an undefined property notice. Replaced !is_null($var->property) with isset($var->property), which ensures the property is checked safely before access.

Testing - UI & Unit tests

root@37cef3b8c11c:/var/www/mdl45-unicat-wr# vendor/bin/phpunit --testsuite mod_observation_testsuite
Moodle 4.5.10+ (Build: 20260327), 57b76e8ee0a041e563e9f42185d77a0897f1ac2e
Php: 8.3.26, pgsql: 17.9 (Debian 17.9-1.pgdg13+1), OS: Linux 6.17.0-1012-oem x86_64
PHPUnit 9.6.18 by Sebastian Bergmann and contributors.

..........................................................        58 / 58 (100%)

Time: 00:13.491, Memory: 113.00 MB

OK (58 tests, 164 assertions)

[Copilot]

Pull request overview

This PR resolves undefined property notices in mod_observation when handling specific form button actions on the session marking page by switching from direct property access via !is_null(...) to safe existence checks with isset(...).

Changes:

• Use isset($fromform->saveandnext) to safely detect the “Save and continue” action.
• Use isset($fromform->abandonbutton) and isset($fromform->submitobservation) to safely detect no-submit button actions (cancel/submit flows).

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