fix(api): keep wiki page name when editing with an empty title (#40153)

Match `PATCH /repos/{owner}/{repo}/wiki/page/{pageName}` document "leave
empty to keep unchanged"

Fixes #39179

---------

Co-authored-by: silverwind <me@silverwind.io>

modules/structs/repo_wiki.go

@@ -43,7 +43,7 @@ type WikiPageMetaData struct {
 // CreateWikiPageOptions form for creating wiki
 type CreateWikiPageOptions struct {
 	// page title. leave empty to keep unchanged
-	Title string `json:"title" binding:"TrimSpace;Required"`
+	Title string `json:"title" binding:"TrimSpace"`
 	// content must be base64 encoded
 	ContentBase64 string `json:"content_base64"`
 	// optional commit message summarizing the change