treewide: use nil value for empty slices

Change-Id: I3a0d9c254841157e6dde1bc21921fdb76db7dde5
Reviewed-on: https://review.monogon.dev/c/monogon/+/2966
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/pkg/tpm/tpm.go b/metropolis/pkg/tpm/tpm.go
index 7d45036..a5a0ee1 100644
--- a/metropolis/pkg/tpm/tpm.go
+++ b/metropolis/pkg/tpm/tpm.go
@@ -285,7 +285,7 @@
 		return []byte{}, fmt.Errorf("sealed data structure is invalid: no sealed key")
 	}
 	// Logging this for auditing purposes
-	pcrList := []string{}
+	var pcrList []string
 	for _, pcr := range sealedBytes.SealedKey.Pcrs {
 		pcrList = append(pcrList, strconv.FormatUint(uint64(pcr), 10))
 	}