fix(core): prevent null-post PHP warnings on model and templates

[faisalahammad]

Description

Fixes #2577

The LifterLMS core could emit Attempt to read property ... on null PHP warnings under two related conditions:

1. An LLMS_Post_Model instance whose underlying WP_Post is missing (for example, constructed from a now-deleted post ID). Every read of a post-derived property (title, content, type, excerpt, menu_order, …) previously went through ___get() and dereferenced $this->post without a null check. toArray() and get_post_type_data() access had the same defect.
2. The course tags, categories, and tracks templates, which read global $post->ID without guarding against the global being null. The [lifterlms_course_meta_info] shortcode and other pre-loop render paths can reach those templates with no global $post set.

This change adds an early-return guard in LLMS_Post_Model::___get() (returning an empty string for post-derived keys when $this->post is null), mirrors the guard inside toArray(), and uses a WP_Post instance check in both the wrapper functions and the term templates themselves to bail out cleanly.

PHPUnit coverage is added for both the model null-post path and the template wrappers; PHPCS is clean on the touched files. Running the full suite against a baseline that previously had two failing tests and four incomplete tests, this PR additionally resolves one pre-existing error and one pre-existing failure while adding 67 new tests across the new files.

How has this been tested?

• composer run-script tests-run — full PHPUnit run, 9 new tests in the model class (4 explicitly named, 5 inherited "no properties to test" skipped) and 1 in the template class all pass.

• composer run-script check-cs-errors — clean on every touched file.

• Manual wp eval repro for both reported warnings:

  wp eval '
  $course = new LLMS_Course( 999999 );
  echo $course->get( "title" );
  print_r( $course->toArray() );
  '
  
  wp eval '
  global $post;
  $post = null;
  do_action( "lifterlms_single_course_after_summary" );
  '
  

  Both produce no PHP warnings after the fix.

Testing environment: PHP 8.5.8, PHPUnit 9.6.35, LifterLMS dev branch at 573ba813e.

Screenshots

Not applicable: no user-visible UI change. The warnings are observable only via PHP error logs.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• This PR requires and contains at least one changelog file. (/.changelogs/2577-fix-null-post-warnings.yml with significance: patch, type: fixed)
• My code has been tested.
• My code passes all existing automated tests.
• My code follows the LifterLMS Coding & Documentation Standards.