[INTERNAL] JSDoc: fix JSDoc issues in the code of the jsdoc-template

 - structure types require an additional set of curly braces
 - optional fields in a structure are denoted with a trailing "="
   as part of the field name

Cherry-picked from SAP/openui5@0cb9f8401.

lib/processors/jsdoc/lib/transformApiJson.js

@@ -157,7 +157,7 @@ function transformer(sInputFile, sOutputFile, sLibraryFile, vDependencyAPIFiles,
 	 *      UI5Types: ["sap.ui.core.Control"] // skip template if its value is "${0}" (default value)
 	 *    }
 	 * @param {string} sComplexType
-	 * @returns {?template: string, ?UI5Types: string[]}
+	 * @returns {{template=: string, UI5Types=: string[]}}
 	 */
 	function parseUI5Types(sComplexType) {
 		let oParsed;