Hyphen isn't texturized if it's the first character

[pento]

If you create a post with the following content:

- here
- is
- a
- test

Then preview or publish it, the hyphen on the first line isn't texturized, while the rest are. The same behaviour occurs in comments, too.

[pento]

It turns out that the same problem occurs at the end of the text block, too.

Anyway, here's a patch.

[pento]

In 31199:

When a hyphen - or double hyphen -- was at the start or the end of a string, it wasn't texturized correctly.

Fixes #31030