The previous method was relying on the idea that product pages would not
trigger the `the_content` filter since they don't have to render the
content. Testing revealed that this idea was unreliable.
In one instance WordPress was triggering `the_content` when attempting
to filter the excerpt, even though the excerpt was empty.
This changes the detection method to a much more direct inspection of
the post's content.
MAILPOET-5859
The current logic for form rendering assumes that a single product page
will trigger the filter `the_content`. This is not always true. In cases
where the product has no description, WooCommerce does not render the
description tab at all, which is where that filter usually fires (in my
testing).
This change ensures that we still give these forms a chance to render on
such pages.
MAILPOET-5859
