When running this plugin on an SVG with an empty block in the CSS, it
would apply it to the matched elements by adding an empty `style`
attribute. See the test for an example.
This resolves that by just dropping the declaration if it's empty.
Detects if a redundant linearGradient or radialGradient is used with
only a single stop, which effectively means a solid color.
If this is found, just remove the gradient and replace references to it
with the color of the first and only stop defined.
