fix(picker): allow empty placeholder on iOS

Author: Almouro

src/withPickerValues/PickerModal.js

@@ -26,7 +26,7 @@ class PickerModal extends PureComponent<PropsType> {
     if (!this.props.values || !this.props.values.length) return null;
     const values = [...this.props.values];
     if (Platform.OS === 'ios') {
-      values.unshift({ value: '', label: placeholder });
+      values.unshift({ value: '', label: placeholder || '' });
     } else {
       // Fix for issue: https://github.com/facebook/react-native/issues/15556
       values.unshift({ value: '', label: '' });