Fix `instrumentForCoverage` for empty `outfiles`

go/tools/builders/cover.go

diff --git a/go/tools/builders/cover.go b/go/tools/builders/cover.go
index e3bbfdd..28fc72d 100644
--- a/go/tools/builders/cover.go
+++ b/go/tools/builders/cover.go
@@ -43,6 +43,9 @@
 		relCoverPath map[string]string,
 		srcPathMapping map[string]string,
 	) ([]string, error) {
+	if len(outfiles) == 0 {
+		return nil, nil
+	}
 	// This implementation follows the go toolchain's setup of the pkgcfg file
 	// https://github.com/golang/go/blob/go1.24.5/src/cmd/go/internal/work/exec.go#L1954
 	pkgcfg := workDir + "pkgcfg.txt"