Port from Gutenberg editor: Fix: Impossible to set empty array on editor-font-sizes.

[jorgefilipecosta]

This ticket ports to core a fix already applied in the Gutenberg editor plugin.
This part of the code is going to be removed from the plugin so it is important to get this change on the core.
Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/13824

More details:
When setting an empty array on editor-font-sizes theme support setting nothing happens, and the default font-sizes were used by the editor.
This patch fixes the problem and makes sure that when add_theme_support('editor-color-palette', array()); is added no available font sizes appear on the editor. This patch uses the same approach used in colors and makes the behavior consistent with what happens when an empty array is set on editor-color-palette.

[SergeyBiryukov]

46290.diff​ looks good to me.

[desrosj]

In 44782:

Editor: Allow the default font size options to be removed.

Similar to editor-color-palette, a theme or plugin should be able to set the theme support for editor-font-sizes to an empty array, indicating that no additional font sizes should be available in the block’s ‘Text Settings’. The current conditional was using empty(), which was incorrectly causing the default font sizes to be used instead.

Props jorgefilipecosta, chrisvanpatten, SergeyBiryukov.
Fixes #46290.

[desrosj]

Reopening for backport consideration.

I was going to note that even after setting editor-font-sizes to array(), the dropdown still "opens". But, that ​seems to be fixed upstream and can be expected in 5.2.

Wondering if we should not backport this so the bug is not visible.

[desrosj]

I previously included the wrong upstream link for the bug I detailed. The correct PR is this one: ​https://github.com/WordPress/gutenberg/pull/13782.

[pbiron]

During the 5.1.1 bug scrub today, it was decided to punt this to 5.2.

[desrosj]

I have tested this after the changes from ​https://github.com/WordPress/gutenberg/pull/13782GB-13782 were merged into trunk in [44808], and the field no longer displays an empty dropdown when an empty array is specified as the value for editor-font-sizes theme support.