Fix resetting to blank value

When you change the `value` prop to a blank string,
Datetime.getSelectedDate() tries to parse the value using the format
string.  This fails for blank values, and then getSelectedDate() falls
back to the previously entered input value from this.state.inputValue,
ignoring the new `value` prop and making the reset fail.

Closes arqex#760.

src/DateTime.js

@@ -520,6 +520,7 @@ export default class Datetime extends React.Component {
 	}
 
 	getInputValue() {
+		if ( this.props.value === '' ) return '';
 		let selectedDate = this.getSelectedDate();
 		return selectedDate ? selectedDate.format( this.getFormat('datetime') ) : this.state.inputValue;
 	}