Bug: margins wrong when using blank line as divider instead of "blank" admonition

[nousernameavailableanymore]

When using a blank line inside a "multi-column" admonition to split a new column, then there is virtually no margin between the columns.

Using

```ad-multi-column
Test 1 
Test 2
Test 3

Test 4
Test 5
```

looks like this.

While using "blank" as divider, columns look like this (which is what it should look like)

```ad-blank
Test 1 
Test 2
Test 3
```
```ad-blank
Test 4
Test 5
```

Both ways should look identical / consistent and behave like the second example (ad-blank) to fully use the page width.