ISSUE: OpenAPITools#17373 [BUG] [Go-server] invalid composite literal type string (OpenAPITools#20467)

* modules/openapi-generator/src/main/resources/go-server: alternative initialization of variable, to avoid extra checks and custom code

* propagated changes to samples

Author: vingarzan

modules/openapi-generator/src/main/resources/go-server/controller-api.mustache

@@ -591,7 +591,7 @@ func (c *{{classname}}Controller) {{nickname}}(w http.ResponseWriter, r *http.Re
 	{{paramName}}Param := r.Header.Get("{{baseName}}")
 	{{/isHeaderParam}}
 	{{#isBodyParam}}
-	{{paramName}}Param := {{dataType}}{}
+	var {{paramName}}Param {{dataType}}
 	d := json.NewDecoder(r.Body)
 	{{^isAdditionalPropertiesTrue}}
 	d.DisallowUnknownFields()