Brief summary of the bug. What is it ? Where is it ?

If the CAMUNDA_OPTIMIZE_IDENTITY_REDIRECT_ROOT_URL property is not set, it defaults to an empty string rather than null and breaks the identity authentication flow.

Steps to reproduce:

Do not set a value for 

CAMUNDA_OPTIMIZE_IDENTITY_REDIRECT_ROOT_URL

Try to access Optimize in CCSM mode

Actual result:

"Invalid redirect_uri"

Expected result:

Authentication is permitted

Notes:

This should be backported to all supported maintenance branches