Pressing return after a list item broken

### Expected
Move the cursor to the beginning of the line of the last list item and tap Enter/Return to create a new line. This should results in this:
```
- Item 1
- Item 2

- Item 4
```

### Observed
This actually results in this:
```
- Item 1
- Item 2

-- Item 4
```

### Reproduced
User report
(Image link: https://d.pr/i/YYGLMt )

Make|Model|iOS Version|App Version
Apple| iPhone 11 | iOS 13.3 |4.14.2
Apple| iPhoneX | OS 13.3 |

Reported in #2654289-zen

Start by reproducing the reported list-editing behavior on the stated iOS versions and compare the expected and observed Markdown output. Done means pressing Return at the beginning of the last list item creates one blank line followed by a single "- Item 4" marker, without duplicating the dash.

Written by the indexing model from the issue text.