Pressing enter too quickly can create broken paragraph blocks

[notnownikki]

Describe the bug

Paragraph blocks should have a single paragraph. It's possible to create paragraphs with line breaks, and break them out into weird, non-blocks.

To Reproduce
Steps to reproduce the behavior:

1. Start a new post with a single paragraph block.
2. Hold down Enter.
3. When you get paragraph blocks that contain more than one line, place the cursor in the middle of one and press enter. You end up with a weird orphaned block with no type.

Expected behavior

All enters sent should split the paragraph block, and we shouldn't have blocks with no type.

Screenshots

[mtias]

Closing in favor of #6021.