CTA Block: Safari crashes when removing all text via backspace

[mindctrl]

Describe the bug
When backspacing to remove all characters in the title and/or button, when you hit the last character Safari goes into a tail spin and beach balls.

Reported here and replicated locally: https://wordpress.org/support/topic/serious-crash-while-editing/

To Reproduce
Steps to reproduce the behavior:

1. Load Safari.
2. Add CTA block to page
3. Type in a title.
4. Backspace until you've removed all characters.
5. See beach ball.

Device and browser information

• macOS Mojave
• Safari 13.0.3