Generate global styles body reset rule prior to processes theme.json

[getdave]

Companion fix to WordPress/gutenberg#36960.

In WordPress/gutenberg#36147 we learn that the Theme JSON class automatically applies a margin: 0 to the body element. Unfortunately it does so in a way that means if you use theme.json to define a margin on the body element, the default margin: 0 will still take precedence.

For example applying this to theme.json will still leave you with the default margin: 0 on the body, rather than the expected value of margin: 0 5rem:

{
	"styles": {
		"spacing": {
			"margin": "0 5rem",
			"padding": "0"
		},
	}
}

This PR fixes this by only applying the default margin: 0 if the root spacing does not have a margin declaration.

Trac ticket: https://core.trac.wordpress.org/ticket/54550

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[github-actions]

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[hellofromtonya]

Committed via https://core.trac.wordpress.org/changeset/52329.