Accept `GIT_VERSION=` environment variable
In some environments it is impossible to unset
environment variable, and no GIT_VERSION= works.
This allows to pass empty value that indicates
that a default should be used. For example allowing
to execute: make git GIT_VERSION=.
Related to: gitlab-compose-kit#59 (closed)
Edited by Kamil Trzciński (Back 2025-01-01)