Make detect empty string before assign image description

[neet]

When I tried to delete image description text, a bug occurred.

see following vid:

this case, even if I'm pressing delete key, the text will re-appear when trying to delete the last character. and this PR fixes it, here's the bug reason:

1. When description changed, component calls handleInputChange() and it sets e.target.value to this.state.dirtyDescription
2. description will be this.state.dirtyDescription or media.get('description') or an empty string
3. When trying to delete last char, this.state.dirtyDescription will be null, so description becomes media.get('description')
4. But media.get('description') still holds description that before I try to delete it.

So it needs to detect whether this.state.dirtyDescription is null or just an empty string.