wp_style_loader_src() expects $_wp_admin_css_colors

[crstauf]

The function wp_style_loader_src() expects that $_wp_admin_css_colors should exist and be an array. This can create a PHP warning if the colors dependency is loaded or style_loader_src filter is used in a context where the global does not exist. Adding checks for the variable will prevent this warning.

Add checks to prevent PHP warning in contexts where colors dependency is printed or style_loader_src is used where $_wp_admin_css_colors is not defined or not an array.

Trac ticket: https://core.trac.wordpress.org/ticket/61302#ticket

[SergeyBiryukov]

In 61388:

Script Loader: Check if $_wp_admin_css_colors is set in wp_style_loader_src().

This aims to avoid PHP warnings if the colors dependency is loaded or the style_loader_src filter is used in a context where the $_wp_admin_css_colors global does not exist.

Follow-up to [7976].

Props crstauf, petitphp, SergeyBiryukov.
Fixes #61302.