[Go] fix isNil check (#17870)

* #17835 changed to return !IsNil

* update samples

---------

Co-authored-by: sagar khot <sagarkhot74@gmail.com>

modules/openapi-generator/src/main/resources/go/model_simple.mustache

@@ -226,7 +226,7 @@ func (o *{{classname}}) Get{{name}}Ok() ({{^isArray}}{{^isFreeFormObject}}*{{/is
 
 // Has{{name}} returns a boolean if a field has been set.
 func (o *{{classname}}) Has{{name}}() bool {
-	if o != nil && {{^isNullable}}!IsNil(o.{{name}}){{/isNullable}}{{#isNullable}}{{#vendorExtensions.x-golang-is-container}}IsNil(o.{{name}}){{/vendorExtensions.x-golang-is-container}}{{^vendorExtensions.x-golang-is-container}}o.{{name}}.IsSet(){{/vendorExtensions.x-golang-is-container}}{{/isNullable}} {
+	if o != nil && {{^isNullable}}!IsNil(o.{{name}}){{/isNullable}}{{#isNullable}}{{#vendorExtensions.x-golang-is-container}}!IsNil(o.{{name}}){{/vendorExtensions.x-golang-is-container}}{{^vendorExtensions.x-golang-is-container}}o.{{name}}.IsSet(){{/vendorExtensions.x-golang-is-container}}{{/isNullable}} {
 		return true
 	}