Fix Farbtastic to make it work with empty input fields, too

[BytesForAll]

Farbtastic requires that the color input field has either a color value or at least a '#' character. If it's empty it won't work. This seems like an artificial limitation and requires workarounds if the integrated Farbtastic is used for input fields that may be empty, i.e. for theme or plugin options where the color is optional. If the color fields has a default value there's no issue but having empty color fields is a useful and valid use case.

This could be fixed in /wp-admin/js/farbtastic.js, line 192

Old:
if (this.value && this.value != fb.color) {

New:
if (this.value != fb.color) {

Looks like WP is working around this for the 'custom background' for themes (which is empty by default) by adding a hardcoded '#' and saving the background-color without '#'

/wp-includes/theme.php, line 1772:
$style = $color ? "background-color: #$color;" : ;

/wp-admin/custom-background.php, line 303:
<input type="text" name="background-color" id="background-color" value="#<?php echo esc_attr(get_background_color()) ?>" />