fix: handle empty git commit in getDeterministicMtime

When building packages in test fixtures or environments without git,
p.C.Git().Commit can be empty. This caused getDeterministicMtime() to
fail when trying to run 'git show -s --format=%ct ""'.

Solution: Return Unix epoch (0) when commit is empty, providing a
deterministic fallback for test environments while maintaining
deterministic behavior.

This fixes CI test failures in TestBuildDockerDeps and
TestBuildDocker_ExportToCache.

Co-authored-by: Ona <no-reply@ona.com>

Author: leodido

pkg/leeway/build.go

@@ -2401,12 +2401,19 @@ func createDockerExportMetadata(wd, version string, cfg DockerPkgConfig) error {
 
 // getDeterministicMtime returns the Unix timestamp to use for tar --mtime flag.
 // It uses the same timestamp source as SBOM normalization for consistency.
+// If no git commit is available (e.g., in test fixtures), returns 0 (Unix epoch).
 func (p *Package) getDeterministicMtime() (int64, error) {
-	timestamp, err := getGitCommitTimestamp(context.Background(), p.C.Git().Commit)
+	commit := p.C.Git().Commit
+	if commit == "" {
+		// No git commit available (e.g., test fixtures) - use Unix epoch for determinism
+		return 0, nil
+	}
+	
+	timestamp, err := getGitCommitTimestamp(context.Background(), commit)
 	if err != nil {
 		return 0, fmt.Errorf("failed to get deterministic timestamp for tar mtime (commit: %s): %w. "+
 			"Ensure git is available and the repository is not a shallow clone, or set SOURCE_DATE_EPOCH environment variable",
-			p.C.Git().Commit, err)
+			commit, err)
 	}
 	return timestamp.Unix(), nil
 }