Fix API not persisting pull request unit config when has_pull_requests is not set (#36718)

The `PATCH /api/v1/repos/{owner}/{repo}` endpoint silently ignores pull
request config fields (like `default_delete_branch_after_merge`,
`allow_squash_merge`, etc.) unless `has_pull_requests: true` is also
included in the request body. This is because the entire PR unit config
block was gated behind `if opts.HasPullRequests != nil`.

This PR restructures the logic so that PR config options are applied
whenever the pull request unit already exists on the repo, without
requiring `has_pull_requests` to be explicitly set. A new unit is only
created when `has_pull_requests: true` is explicitly sent.

Fixes https://github.com/go-gitea/gitea/issues/36466
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

routers/web/repo/issue_view.go

@@ -905,9 +905,8 @@ func preparePullViewReviewAndMerge(ctx *context.Context, issue *issues_model.Iss
 	// Check correct values and select default
 	if ms, ok := ctx.Data["MergeStyle"].(repo_model.MergeStyle); !ok ||
 		!prConfig.IsMergeStyleAllowed(ms) {
-		defaultMergeStyle := prConfig.GetDefaultMergeStyle()
-		if prConfig.IsMergeStyleAllowed(defaultMergeStyle) && !ok {
-			mergeStyle = defaultMergeStyle
+		if prConfig.IsMergeStyleAllowed(prConfig.DefaultMergeStyle) && !ok {
+			mergeStyle = prConfig.DefaultMergeStyle
 		} else if prConfig.AllowMerge {
 			mergeStyle = repo_model.MergeStyleMerge
 		} else if prConfig.AllowRebase {