tform_base.inc.php does not repsect default value

When you add a new form field and set a 'default' value, that value does not show up in your form.

I've noticed this in the past when I added fields but did not find the time then to dig into it.

When I did is was actually quite obvious ... the getHTML() in tform_base just takes an empty string as default.

See my MR for a fix.