fix: Don't overwrite source_name if source_name is defined (bp #12280) (#12341)

Co-authored-by: Richard Case <64409021+casesolved-co-uk@users.noreply.github.com>
Co-authored-by: Suraj Shetty <13928957+surajshetty3416@users.noreply.github.com>

frappe/public/js/frappe/model/create_new.js

@@ -362,9 +362,7 @@ $.extend(frappe.model, {
 			);
 		} else if (!opts.source_name && opts.frm) {
 			opts.source_name = opts.frm.doc.name;
-
-			// Allow opening a mapped doc without a source document name
-		} else if (!opts.frm) {
+		} else if (!opts.frm && !opts.source_name) {
 			opts.source_name = null;
 		}