createBlock fires error if name doesn't match a registered block type.

[ryanwelcher]

Describe the bug
When calling createBlock() with a block name that is not registered, a console error is fired due to blockType being undefined when accessing the .attributes property here

To reproduce
Steps to reproduce the behavior:

1. Create a new Post or Page
2. In the console panel call wp.blocks.createBlock('not/registered)
3. See error in console.

Expected behavior
I would expect this function to fail with a console warning of Block type {name} is not registered.

Screenshots

Editor version (please complete the following information):

• WordPress version: 5.4.2
• No plugin installed.

Desktop (please complete the following information):

• OS: Mac OS 10.15.6
• Browser Chrome
• Version 84.0.4147.105