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/logtree/zap.go b/metropolis/pkg/logtree/zap.go
index 7790cd0..7fd364a 100644
--- a/metropolis/pkg/logtree/zap.go
+++ b/metropolis/pkg/logtree/zap.go
@@ -126,7 +126,7 @@
 	delete(entry, "level")
 	delete(entry, "time")
 	delete(entry, "caller")
-	extra := []byte{}
+	var extra []byte
 	if len(entry) > 0 {
 		extra, _ = json.Marshal(entry)
 	}
diff --git a/metropolis/pkg/tpm/eventlog/internal/events.go b/metropolis/pkg/tpm/eventlog/internal/events.go
index f41ed1c..78d305c 100644
--- a/metropolis/pkg/tpm/eventlog/internal/events.go
+++ b/metropolis/pkg/tpm/eventlog/internal/events.go
@@ -277,8 +277,8 @@
 	}
 	signatures := efiSignatureList{}
 	buf := bytes.NewReader(b)
-	certificates := []x509.Certificate{}
-	hashes := [][]byte{}
+	var certificates []x509.Certificate
+	var hashes [][]byte
 
 	for buf.Len() > 0 {
 		err := binary.Read(buf, binary.LittleEndian, &signatures.Header)
@@ -368,7 +368,7 @@
 }
 
 func parseEfiSignature(b []byte) ([]x509.Certificate, error) {
-	certificates := []x509.Certificate{}
+	var certificates []x509.Certificate
 
 	if len(b) < 16 {
 		return nil, fmt.Errorf("invalid signature: buffer smaller than header (%d < %d)", len(b), 16)
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))
 	}