Fix panic in debug logging (#7702)

Fix a panic when res.Digest is nil

server/remote_cache/digest/digest.go

@@ -547,8 +547,15 @@ func MissingDigestError(d *repb.Digest) error {
 	}
 }
 
-// String returns the digest formatted as "HASH/SIZE".
+// String returns the digest formatted as "HASH/SIZE" or the string "<nil>"
+// if the digest is nil.
+//
+// Note: this is intended mainly for logging - to get a representation of a
+// digest suitable for use as a map key, use NewKey instead.
 func String(d *repb.Digest) string {
+	if d == nil {
+		return "<nil>"
+	}
 	return fmt.Sprintf("%s/%d", d.Hash, d.SizeBytes)
 }