fix: update enable_prune even if mirror_interval is not provided (go-gitea#28905)

Currently, the `updateMirror` function which update the mirror interval
and enable prune properties is only executed by the `Edit` function. But
it is only triggered if `opts.MirrorInterval` is not null, even if
`opts.EnablePrune` is not null.

With this patch, it is now possible to update the enable_prune property
with a patch request without modifying the mirror_interval.

## Example request with httpie

### Currently:
**Does nothing**
```bash
http PATCH https://gitea.your-server/api/v1/repos/myOrg/myRepo "enable_prune:=false" -A bearer -a $gitea_token
```

**Updates both properties**
```bash
http PATCH https://gitea.your-server/api/v1/repos/myOrg/myRepo "enable_prune:=false" "mirror_interval=10m" -A bearer -a $gitea_token
```

### With the patch
**Updates enable_prune only**
```bash
http PATCH https://gitea.your-server/api/v1/repos/myOrg/myRepo "enable_prune:=false" -A bearer -a $gitea_token
```

Author: Anthony-Jhoiro

routers/api/v1/repo/repo.go

@@ -637,7 +637,7 @@ func Edit(ctx *context.APIContext) {
 		}
 	}
 
-	if opts.MirrorInterval != nil {
+	if opts.MirrorInterval != nil || opts.EnablePrune != nil {
 		if err := updateMirror(ctx, opts); err != nil {
 			return
 		}