Dropdown clearable sets '' as empty value.

[dominikdosoudil]

Dropdown sets '' as value when it's cleared using cross button. I think that much more sense would make null as it's meaning is literally empty value. Moreover '' makes even less sense if e.g. numbers are used as keys of other options.

https://github.com/Semantic-Org/Semantic-UI-React/blob/21cce97ec4b9e7d46615a18b18cc2cd7dce7a0a9/src/modules/Dropdown/Dropdown.js#L772

Using null as an empty value should also resolve https://github.com/Semantic-Org/Semantic-UI-React/issues/1748