age: improve error on empty files

Fixes #416

cmd/age/testdata/x25519.txt

@@ -15,6 +15,11 @@ age -r age12phkzssndd5axajas2h74vtge62c86xjhd6u9anyanqhzvdg6sps0xthgl -o test.ag
 ! age -d -i key.txt test.age
 stderr 'no identity matched any of the recipients'
 
+# decrypt an empty file
+! age -d -i key.txt empty
+stderr empty
+
+-- empty --
 -- input --
 test
 -- key.txt --

internal/format/format.go

@@ -239,7 +239,9 @@ func Parse(input io.Reader) (*Header, io.Reader, error) {
 	rr := bufio.NewReader(input)
 
 	line, err := rr.ReadString('\n')
-	if err != nil {
+	if err == io.EOF {
+		return nil, nil, errorf("file is empty")
+	} else if err != nil {
 		return nil, nil, errorf("failed to read intro: %w", err)
 	}
 	if line != intro {