[Rating] Simpler customization of active "no value" styles

[eps1lon]

BREAKING CHANGE:
Customization of pristine label

-input:focus + .MuiRating-pristine {
+.MuiRating-labelEmptyValueActive {
    top: 0;
    bottom: 0;
    position: absolute;
    outline: 1px solid #999;
    width: 100%;
}

Preliminary change which isn't required for this change but aids communication of intent: Replace pristine with empty value.
Pristine is an existing term in forms and means "untouched" or "unchanged". Right now keep using a class named pristine even if the user has interacted with the input. This can be confusing.

Actual change:
We usually try to use class names for styling. This allows easier customization via classes API and reduces specificity wars. However, previously styling of the Rating when no rating was given required a combination of class name, sibling and pseudo class selector i.e. knowledge of implementation. This change introduces a single class (labelEmptyValueActive) which replaces the old selector. It requires more JS for the styling but less for the label-value association since we can now rely on association by hierarchy instead of by ID.

[mui-pr-bot]

Details of bundle changes

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