VM3 customfields / $field->display / output problem with stringlists

[conejo]

Hi together,
I finish my update from J2.5/VM2 to J3/VM3 and after some template changes is working almost fine.
I have a problem displaying a customfield which is defined as a list.
This customfield is defined as string and it "is a list" is active.
The default value (to do a list) is: ;item1;item2;item3
The customfield is attached to a product and no item for this customfield is selected.
It shows: "Select an Option".

Under VM2 the $field->display output is an empty string.
Now under VM3 the $field->display output is the list as text exactly as defined in default value: ";item1;item2;item3;"

What can I do to get an empty string again?

Many thanks for your help,

Alejandro