Protect against bad data in processing theme.json

[oandregal]

Steps to reproduce the bug:

• edit lib/global-styles.php and add a new context that doesn't represent any block's selector. For example, by including a typo such as:

"core/paragrah": { // Note the missing 'p'.
    "settings": {
        "color": {
            "custom": false
        }
    }
}

• load the front-end of the site and see the error message (you may have to activate WP_DEBUG).