[change] Set default site to empty instead of removing it

[nemesifier]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• N/A: I have written new test cases for new code and/or updated existing tests for changes to existing code.
• N/A: I have updated the documentation. N/A

Description of Changes

This should avoid the default site to be overridden when upgrading the Freeradius package, which can cause the openwisp site to break.