Remove potential false positive assertions (#33288)

Remove potential false positive assertions (#33288)

* Remove potential false positive assertions

querySelector() returns null but

expect(document.querySelector('...')).toBeDefined()

tests that the value is not undefined

* Migrated assertions from .toBeDefined() to .not.toBeNull() for .getInstance() calls in tests

* Migrate offcanvas assertions from .toBeDefined() to .not.toBeNull() for .getInstance() call

* convert more cases to not.toBeNull assertions

Co-authored-by: XhmikosR <xhmikosr@gmail.com>