Fix invalid issue branch reference if not specified in template (#22513) (#22520)

Backport #22513

Co-authored-by: Brecht Van Lommel <brecht@blender.org>

routers/web/repo/issue.go

@@ -786,7 +786,8 @@ func setTemplateIfExists(ctx *context.Context, ctxDataKey string, possibleFiles
 			}
 
 		}
-		if !strings.HasPrefix(template.Ref, "refs/") { // Assume that the ref intended is always a branch - for tags users should use refs/tags/<ref>
+
+		if template.Ref != "" && !strings.HasPrefix(template.Ref, "refs/") { // Assume that the ref intended is always a branch - for tags users should use refs/tags/<ref>
 			template.Ref = git.BranchPrefix + template.Ref
 		}
 		ctx.Data["HasSelectedLabel"] = len(labelIDs) > 0