Blank page after deleting all text in the text box

[leftiness]

Steps to reproduce:

1. Open the Mermaid live editor
2. Delete all text in the text box

Outcome:

• Blank page. No text box. No page header. Nothing. React root is empty.

Expectation:

• The chart should be empty, but the rest of the editor should remain.

Notes:

• Tested in Firefox 54.0 and Chromium 56.0.2924.76 on Ubuntu 16.04

[tylerlong]

Fixed. Thanks.