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>

modules/optional/option.go

@@ -67,3 +67,17 @@ func ParseBool(s string) Option[bool] {
 	}
 	return Some(v)
 }
+
+func AssignPtrValue[T comparable](changed *bool, target, src *T) {
+	if src != nil && *src != *target {
+		*target = *src
+		*changed = true
+	}
+}
+
+func AssignPtrString[TO, FROM ~string](changed *bool, target *TO, src *FROM) {
+	if src != nil && string(*src) != string(*target) {
+		*target = TO(*src)
+		*changed = true
+	}
+}