Allow gitlab_project_protected_environment.group_inheritance_type to be zero

This change fixes how the system handles a setting called "group inheritance type" for GitLab project protected environments. Previously, the code was incorrectly ignoring this setting when its value was 0 (zero), treating it as if no value was provided. Now the code properly accepts and processes zero as a valid value, which allows users to explicitly set this inheritance type to 0 when needed. The fix applies to both creating new protected environments and updating existing ones, and includes a test to verify that setting the value to 0 works correctly.