How to set default value in environment variable in nginx conf file

[nvignesh86]

CLI Version: 8.7

Build pack 1.25.5

We are using enviornment value in nginx conf file. some how environment variable empty or null scenario build is failing.
because of "gzip [Not setting any value]". nginx conf format wrong.

Need to give default value for GZIP_DOWNLOADS while fetching in nginx.conf.
Can you please let us know how to give default value for.

Expect to happen
Build should be success even environment variable is not setting.

Actual behavior
build is failing in nginx.conf file.