Replace assert_null and assert_not_null with assert_equals and assert_not_equals.

Without this change,
html-templates/parsing-html-templates/clearing-the-stack-back-to-a-given-context/clearing-stack-back-to-a-table-body-context.html
and
html-templates/parsing-html-templates/clearing-the-stack-back-to-a-given-context/clearing-stack-back-to-a-table-row-context.html
fails with exceptions.