Field sync with Jira flag needed

Currently with a one-way sync from ZD to Jira, the fields are set to sync.

ie

Ticket 123
 
Fields:
 
Client URL: https://google.com 
Specific Screen URL:https://google.com/photos
 
In Jira:
 
Jira issue: 345
 
Fields:
 
Client URL: https://google.com
Specific Screen URL:https://google.com/photos
 
If, for any reason, the field in the Zendesk ticket 123 is changed from:
 
Client URL: https://google.com
Specific Screen URL:https://google.com/photos
 
To 

ZD
Client URL: null
Specific Screen URL: null

It will overite the Jira fields to 

Client URL: null
Specific Screen URL: null

I'd like to see added that if the ZD fields are not changed (Null), they should not overwrite the field in Jira. You can use a flag trigger if only the field is changed, not what it is.  Otherwise, the field sync is useless to me, and I don't want to go back into ZD each time if it is missed.

ZD
Client URL: null
Specific Screen URL: null

Leave alone if no change is made to the ZD ticket fields.

Client URL: https://google.com
Specific Screen URL:https://google.com/photos