git: add default value null to programs.git.signing.key

[folliehiyuki]

Description

Right now, if other signing options are defined without key specified, home-manager switch will fail.

Checklist

• Change is backwards compatible.

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Maintainer CC

@rycee

[Adda0]

Seems reasonable to me. When the key is null, GnuPG will decide what signing key to use based on the author of the commit, which in my opinion should be the default behaviour. When one wants to overwrite that, they can set key to a specific signing key.

[teto]

LGTM. Can you rebase to fix CI plz (can't merge without green CI)