Fix panic when git config contains a key without a value

[LemonCondor]

What happened

git-sync can panic during startup when git config list -z
returns a configuration key without a value.

The parser assumes every entry contains a \n separator and
unconditionally accesses the second element after SplitN.

Git can emit a key without a value as key\0, without that separator.

What this changes

Handle entries without a value explicitly and preserve them in the
diagnostic output as null.

This keeps them distinct from explicitly empty values ("").

Testing

• go test ./...
• go vet ./...
• gofmt
• git diff --check

The regression test covers keys without values, empty values,
multi-line values, normal key/value entries, and multiple entries.

Fixes #1005

AI disclosure

AI tools were used during the development and analysis of this
contribution. The final patch, test, and this description have been
reviewed by the contributor, who takes responsibility for the submitted
changes.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: high-altitude-ai / name: Guillaume Marty (c17a0e3)

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[thockin]

Thanks!

/lgtm
/approve

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