Don't store signatures if there is none of them

Currently, the copy command prints the message 'Storing signatures' and
calls the signature storing function, even if there are no signatures
present. This can mislead users and make them believe that there are
image signatures.

The proposed change modifies the copy function to print the message and
invoke the image storing function only if there is at least one signature.

Signed-off-by: Mike <mike.sul@foundries.io>

copy/single.go

@@ -256,9 +256,11 @@ func (c *copier) copySingleImage(ctx context.Context, policyContext *signature.P
 	}
 	sigs = append(sigs, newSigs...)
 
-	c.Printf("Storing signatures\n")
-	if err := c.dest.PutSignaturesWithFormat(ctx, sigs, targetInstance); err != nil {
-		return nil, "", "", fmt.Errorf("writing signatures: %w", err)
+	if len(sigs) > 0 {
+		c.Printf("Storing signatures\n")
+		if err := c.dest.PutSignaturesWithFormat(ctx, sigs, targetInstance); err != nil {
+			return nil, "", "", fmt.Errorf("writing signatures: %w", err)
+		}
 	}
 
 	return manifestBytes, retManifestType, retManifestDigest, nil