Return an empty digest where the Parser validation fails

Signed-off-by: Nima Talebi <[email protected]>

digest.go

@@ -80,7 +80,10 @@ var (
 // be returned if the format is invalid.
 func Parse(s string) (Digest, error) {
 	d := Digest(s)
-	return d, d.Validate()
+	if err := d.Validate(); err != nil {
+		return "", err
+	}
+    return d, nil
 }
 
 // FromReader consumes the content of rd until io.EOF, returning canonical digest.