Add support for [current-page:post:*].

I'm not sure how the dynamic tokens are supposed to work, but if there are no post values matched, the token itself prints as a string inside the webform field e.g. [current-page:post:email].

This patch makes a simple change to print an empty string if the POST value doesn't exist, as seeing the token itself is not useful. I see the same happens for the "query" dynamic token, which is also arguably not expected behaviour for users. Thoughts?