forms: Fix crash if selection-field not set

If a custom list item were added to an email template, and there is no data
for the field (ie. the field is not required) when the template is rendered,
the system would crash.

toString() is required to return a string, no matter what. This patch
cascades the (string) coercion to the current value of the list field if it
is not an instance of a DynamicListItem