Theme JSON: remove redundant check and relocate $selectors assignment

[ramonjd]

Backporting:

• Theme JSON: remove redundant check and relocate $selectors assignment gutenberg#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

Trac ticket: https://core.trac.wordpress.org/ticket/62234

[github-actions]

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Props ramonopoly, mukesh27.

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[github-actions]

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[mukeshpanchal27]

@ramonjd Good if we commit for 6.7. WDYT?

[ramonjd]

Good if we commit for 6.7. WDYT?

Thank you, yes!

[ramonjd]

Updated with code suggestion. Ready for final ✔️

[mukeshpanchal27]

🎉

[mukeshpanchal27]

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

[ramonjd]

Committed in r59262 / 1508e5b