Remove <style> element from the minifyStyles plugin when it is empty

[yisibl]

Fixed: #1226

In the minifyStyles plugin, if the <style> is empty or the css rule in it is not applied to the current svg node, we should remove the empty <style> element.

A similar process has been done in the inlineStyles plugin.

svgo/plugins/inlineStyles.js

Lines 220 to 223 in 091172a

	if (style.cssAst.children.isEmpty()) {
	// clean up now emtpy <style/>s
	var styleParentEl = style.styleEl.parentNode;
	styleParentEl.spliceContent(styleParentEl.content.indexOf(style.styleEl), 1);

Origin

<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
    <style></style>
    <style />
    <rect x="10" y="10" width="100" height="100"/>
</svg>

Before:

<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
    <style />
    <style />
    <rect x="10" y="10" width="100" height="100"/>
</svg>

After:

<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
    <rect x="10" y="10" width="100" height="100"/>
</svg>

[yisibl]

M46 24a21.9 21.9 is invalid path data, fixed it.

[yisibl]

At present, the test here fails, and it actually generates:

<style>
</style>

I don't know how to solve it yet.

[strarsis]

My two cents here: Wouldn't it make sense to add a plugin that just removes empty <style> elements?
As multiple plugins should now implement the same logic, it should make sense to put this into a dedicated plugin.

Related: #592 (comment)

[TrySound]

@strarsis I think #1381 perfectly fits into this task since it combines multiple <style> content it may collapse <style> into none.

[yisibl]

@strarsis If there can be better done, I support it.

[TrySound]

This is implemented as part of new mergeStyles plugin. Thanks anyway!

[yisibl]

Continue to improve: #1457