fix: empty ModelExample rendering in a Response w/o content (via #5504)

* add failing test

* use falsy notSetValue for oas3SchemaForContentType

test/e2e-cypress/static/documents/bugs/5453.yaml

@@ -0,0 +1,10 @@
+openapi: 3.0.2
+info:
+  title: Response without a schema
+  version: 1.0.0
+paths:
+  /foo:
+    get:
+      responses:
+        '200':
+          description: OK

test/e2e-cypress/tests/bugs/5453.js

@@ -0,0 +1,11 @@
+// http://github.com/swagger-api/swagger-ui/issues/5453
+
+  describe("#5453: Responses w/o `content` should not render ModelExample", () => {
+   it("should not render a ModelExample section", () => {
+     cy.visit("/?url=/documents/bugs/5453.yaml")
+       .get("#operations-default-get_foo")
+       .click()
+       .get(".responses-inner")
+       .should("not.have.descendants", ".model-example")
+   })
+ })