Disabling the color palette and keeping the gradients palette results in a broken inspector panel

In the buttons block (and cover) (all blocks supporting gradients), we should be able to disable the color palette and keep the gradients by adding this to your theme functions.php

```
	add_theme_support( 'editor-color-palette', array() );
```

right now, if you do so, you end up with a broken inspector panel.

cc @jorgefilipecosta 