fix: use null as a notSetValue for examplesForMediaType (via #5503)

* fix: use null as a notSetValue for examplesForMediaType

fixes #5455

* tests!

test/e2e-cypress/tests/bugs/5455.js

@@ -0,0 +1,11 @@
+// http://github.com/swagger-api/swagger-ui/issues/5455
+
+describe("#5455: Request bodies w/o `examples` should not render a dropdown", () => {
+  it("should not render a <select> element", () => {
+    cy.visit("/?url=/documents/bugs/5455.yaml")
+      .get("#operations-default-post_foo")
+      .click()
+      .get(".opblock-section-request-body > .opblock-description-wrapper")
+      .should("not.have.descendants", "select")
+  })
+})