Preserve existing contact details when updating from pre-chat form

Problem
When customers fill out (or skip) pre-chat form fields, those values overwrite existing contact information even if the new data is blank or incorrect. This corrupts known-good contact details.

Request
Don't overwrite existing contact information with new pre-chat form data unless the customer explicitly provides new values.

Context
Pre-chat forms are needed to help identify customers, but they shouldn't corrupt existing contact records.