Be more verbose if there are no metadata found

Just notice this after running optipng -strip all -i 0 -o 9 mat2.png for !20 (merged)

Before:

mat2 -s mat2.png
[+] Metadata for mat2.png:
  SignificantBits: 8 8 8 8
  PixelsPerUnitX: 15118
  PixelsPerUnitY: 15118
  PixelUnits: meters
  Software: www.inkscape.org
  Title: Adwaita Icon Template
  Author: GNOME Design Team
  Copyright: CC Attribution-ShareAlike http://creativecommons.org/licenses/by-sa/4.0/

After:

mat2 -s mat2.png
[+] Metadata for mat2.png:

Maybe output something along the lines of "No metadata found" instead of just nothing