Describe the bug
Currently, a Button block without a title attribute still renders an empty button via the save function, leaving an empty <a> on the page.

To reproduce
Steps to reproduce the behavior:

1. Add an empty Button block
2. Publish the page
3. View the published page
4. See error

Expected behavior
A button is not rendered unless the title attribute is entered, preventing empty buttons from displaying if it's not intentionally configured within the editor.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: MacOS
• Browser: Safari
• Version: 12.1.1

Additional context
Using Gutenberg 6.3, although it happens regardless of whether or not the plugin is active.