Color picker field wrongly returns rgba(0,0,0,0) for unset values

The color picker returns rgba(0,0,0,0) if the field value is unset. This is especially curious behaviour since the field value can be cleared using a dedicated "clear" button in the field UI.

The issue is easily reproducible, and the code leading to this misbehaviour is obvious in the string_to_array() function in class-acf-field-color_picker.php: If $value is empty, all the conditions fall through and rgba(0,0,0,0) is returned.

IMNSHO, the field should return false or null if the value is not set, in line with other fields. Alternatively, one could argue it should respect and return the default value if set. In any case, returning rgba(0,0,0,0) is wrong since it is not a null value but an actual color and especially wrong if one only were only to take the RGB components returned from an alpha-less color picker.

Edit: Of course one can test for rgba(0,0,0,0) and treat it as "unset" value down the line, but it still shouldn't return an rgba value in the first place, even if it's an obviously "wrong" one.

Start in class-acf-field-color_picker.php at the string_to_array() function and reproduce the unset-value case described in the issue. Review how other fields represent unset values and determine the expected null or false behavior, including whether a configured default should apply. Update the relevant behavior and verify that an unset color no longer becomes rgba(0,0,0,0).