SVGLint exits with zero code when no files are found

[mondeja]

svglint my-non-existent-file.svg shows a summary

------------------------------------------------------------------------------- Summary -------------------------------------------------------------------------------
- No files linted

and exits with zero code.

This doesn't makes any sense as could lead to ignored checks in pipelines. Given the addoption of the tool, even I would consider it as a bug, let's make it a breaking change so as not to cause unexpected surprises.