I added a new evar( customer id) to the Report Suite in production but the rule that will hit this evar is not active yet. I noticed that this evar has undefined value for each beacon has been sent to the Report Suite. How can I avoid to populate this new evar with undefined value until i dont bring in production the rule will hit this evar?
You can't avoid it. Read the following article to understand what the undefined value means.
An event fires without a conversion variable: For example, a user comes to your site and makes a purchase without any eVar1 value. If you view orders using the eVar1 dimension, there is no value to attribute this order to. Therefore, it is automatically attributed to "Unspecified".
"I understand the reason of undefined value, what is your advise when we add a new evar in DEV Report Suite and PROD Report Suite but the rule will hit the evar in the PROD Report Suite will bring in PROD later?"
The recommendation is to continue testing and sending data to only the DEV report suite. After the changes are verified, the rule should be published to production with the evar hitting and sending data to PROD report suite. It will minimize the "Unspecified" instances.
You can enable the evar when you are ready to push data into it but you will still get Unspecified for evar(if persistence is anything other than hit) against metrics like Visits, UVs or any custom/standard events except for instances of that evar.
As per the design of conversion variable (evar) report there can be 2 cases when Unspecified appears in the report:
A conversion event fires without a conversion variable
Hits where conversion variable was not present
#1 can be handled if the event is fired in the same server call as the variable however, there would still be some server calls during the visit, where evar is not expected to fire. All such hits will be grouped under Unspecified in eVar report.
In your case because of #2 you are getting Unspecified in the eVar reports. If you want to remove 'Unspecified' just from reports, you can simply click on the filter icon next to dimension and uncheck the 'Show Unspecified' checkbox.