Custom gradient cannot be edited if there are no explicit stops specified.

[properlypurple]

Describe the bug
When declaring custom gradient presets, if a gradient is described without explicit stops, it's not possible to edit it in the gradient control in the editor.

To reproduce
Steps to reproduce the behavior:

1. Add this code to your theme

add_theme_support( 
	'editor-gradient-presets', 
	array(
		array(
		'name'     => __( 'Gradient', 'themeslug'  ),
		'gradient' => 'linear-gradient(to bottom, #000000, #123456 )',
		'slug'     => 'new-gradient'
		)
	)
);

2. Edit a post/page, and select a block which supports gradients
3. Try selecting the gradient described in the code above
4. No color stops appear in the gradient bar control.

Expected behavior
When no explicit stops are defined, the control should calculate them automatically and insert color stops.

Screenshots

Editor version (please complete the following information):

• WordPress version: 5.4.2
• Does the website has Gutenberg plugin installed, or is it using the block editor that comes by default? : Gutenberg plugin
• If the Gutenberg plugin is installed, which version is it? 8.4

Desktop (please complete the following information):

• OS: Archlinux
• Browser: Firefox
• Version: 77.0.1