[Fonts API] Remove global from wp_print_fonts()

[hellofromtonya]

Partially fixes #50075.
Sets up decision making for which fonts to print for PR #50151.

What?

Removes the global check. Instead checks if any fonts are registered; if no, bails out.

Why?

The original design (being replaced by this PR) comes from wp_print_styles().

The global check exists to avoid instantiating WP_Fonts when invoking wp_print_fonts(). In other words, if wp_print_fonts() is called before the API is used, it would bail out. This technique saves the overhead within wp_fonts(), i.e. instantiating WP_Fonts and registering the local provider. However, this overhead is tiny and, in itself, not worthy of having a global variable for this purpose.

The function lacks a proper bail out check: if there are no fonts registered, then there's nothing to print. The global check assumed fonts are registered; however, this is a false assumption.

How?

Removes the global check.

Replaces it with wp_fonts()->get_registered_font_families(). If it returns empty, then there are no fonts registered to print. Even if handles are passed into the wp_print_fonts(), those handles must be registered.

Checking if no fonts are registered:

• is a simple, effective first-tier approach. More checks exist with WP_Fonts::do_items() stack.
• sets up the function for printing all registered fonts when in the Site Editor (see [Fonts API] Relocate which fonts to print from script-loader into wp_print_fonts() #50140).

Testing Instructions

Visually Test:

Set up on your test site:

1. Activate the Gutenberg plugin.
2. Activate TT3 theme.

Before applying this PR:

1. Go to the Site Editor > Styles > Typography.
2. Using your browser's dev tools, search the HMTL for wp-fonts-local <style> element.
3. Copying the internals of that element. You'll use this to compare after applying this PR.

After applying this PR:

1. Go to the Site Editor > Styles > Typography.
2. Using your browser's dev tools, search the HMTL for wp-fonts-local <style> element.
3. Compare the styles to the before styles from above. They should match.

To test the early bail out if no fonts are registered:

1. Add the following code to wp-content/mu-plugins/test.php:

<?php

var_dump( wp_print_fonts() );
exit;

An empty array should be displayed in your browser.

Running the automated tests

All tests should pass when running the PHPUnit tests:

npm run test:unit:php -- --filter Tests_Fonts_WpPrintFonts

[anton-vlasenko]

The code looks good to me.
I have tested this PR, and the contents of the style#wp-fonts-local elements is the same before and after the patch.
Therefore, I approve this PR.

[bph]

I just cherry-picked this PR to the release/15.7 branch to get it included in the next release: c922336