Theme JSON: remove redundant check and relocate $selectors assignment in get_block_nodes()

[ramonopoly]

A ticket to track the backport of Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/66154

In WP_Theme_JSON::get_block_nodes(), remove the redundant check for $theme_json['styles'], which means WP_Theme_JSON::get_blocks_metadata() is only called if necessary.

This also has the potential to slightly improve the performance of functions such as [wp_add_global_styles_for_blocks](​https://github.com/wordpress/wordpress-develop/blob/trunk/src/wp-includes/global-styles-and-settings.php#L254) that call WP_Theme_JSON::get_styles_block_nodes(), where no block styles exist.

Backporting:

• ​https://github.com/WordPress/gutenberg/pull/66154

## What? Why? How?

In WP_Theme_JSON::get_block_nodes(), Remove redundant check for $theme_json['styles'], which means WP_Theme_JSON::get_blocks_metadata() is only called if necessary.

This also has the potential to slightly improve the performance of functions such as ​wp_add_global_styles_for_blocks that call WP_Theme_JSON::get_styles_block_nodes(), where no block styles exist.

## Testing Instructions
CI tests should pass.
Smoke test in a block theme with lots of block styles, like TT4 and a theme with none, like Emptytheme

@ramonjd Good if we commit for 6.7. WDYT?

Good if we commit for 6.7. WDYT?

Thank you, yes!

Updated with code suggestion. Ready for final ✔️

@ramonjd Move into 6.7 and commit when you have moment.

[ramonopoly]

In 59262:

Theme JSON: remove redundant styles check and relocate $selectors assignment.

In WP_Theme_JSON::get_block_nodes(), remove redundant check for $theme_json['styles'], which means WP_Theme_JSON::get_blocks_metadata() is only called if necessary.

Also skip unnecessary $selector assignment if only nodes are to be returned.

Props ramonopoly, mukesh27.
Fixes #62234.

Committed in r59262 / 1508e5bf0fa3ad2d001de08eab4dee49985bb322