`New Pull Request` Should not be suggested for Single Branch Repository

[yekanchi]

Description

• When There is only One Branch There Should Not be Suggested to Create New Pull Request. it's useless.
• Allow To Close Pull Request Dialog. it is annoying when you don't want to create new pull request, a tiny close button is useful.

Screenshots

Gitea Version

1.21.0+dev-266

How are you running Gitea?

Locally

[lunny]

This should be fixed by #25795, duplicated with #25778