Rewrite value using another field value only if empty.

Problem/Motivation

Need to rewrite only when field is empty.

Proposed resolution

Check if the field is empty, if it is then use another field(s) value or even a default value.
Rewrite plugin has the flexibility to use multiple replacement patterns, custom string, or a mix of both.
Include the conditional to do the rewrite only if the field value is empty or it is not present in the source.

User interface changes

include 'only if empty' checkbox