Allow empty specialValueText in QAbstractSpinBox

Special value text functionality is now switched on by providing non-empty specialValueText. In most situations the most appropriate specialValueText is actually an empty string. But it cannot be used since that would disable the special value text functionality altogether.

It is therefore needed to use some artifical special text for this purpose, like " ", "null" or "-" to represent null value in the spinbox. But then, it is too difficult for the user to set this value because: 1) the user has to know what text to enter; 2) the user has to write the special text. Just deleting the present value would be more convenient for the user.

It is also not possible to use QLineEdit::placeholderText functionality in a spinbox because the spinbox cannnot be empty even if it logically contains an empty value.