When adding images to a Gallery block, the button on the final step has an empty string inside it, instead of Insert into Post.

To Reproduce

1. Add a Gallery block
2. Click on Media Library button
3. Click on the Media Library tab (if not already selected)
4. Select multiple images
5. Click on Create a new gallery button

Screenshots

Desktop (please complete the following information):

• OS X 10.13.6
• Firefox 63
• Gutenberg 3.8.0