[GSoC PATCH] submodule: warn on valueless active config

Hi everyone,

My name is Tilak  (he/him), and I am a second-year Electronics and
Instrumentation Engineering student at NIT Rourkela. I am preparing to
apply for GSoC 2027 and am starting my contributions to Git.

Regarding my background with Git: I have built Git from source,
successfully
navigated the codebase, and tackled the NEEDSWORK comment regarding
valueless 'submodule.active' configurations in submodule.c.

Below is my microproject patch resolving this issue by switching from
repo_config_get_string_multi() to repo_config_get_value_multi() and
adding an automated test case in t7400-submodule-basic.sh.

I look forward to your feedback!

Thanks,
Tilak