Fix OpenAPIGen for opt payloads (zio#3181)

zio-http/jvm/src/test/scala/zio/http/endpoint/openapi/OpenAPIGenSpec.scala

@@ -1029,8 +1029,7 @@ object OpenAPIGenSpec extends ZIOSpecDefault {
                          |                  {
                          |                    "$ref" : "#/components/schemas/Payload"
                          |                  }
-                         |                ],
-                         |                "description" : ""
+                         |                ]
                          |              }
                          |            }
                          |          },

zio-http/shared/src/main/scala/zio/http/endpoint/openapi/JsonSchema.scala

@@ -55,11 +55,11 @@ private[openapi] case class SerializableJsonSchema(
     if (nullable && schemaType.isDefined)
       copy(schemaType = Some(schemaType.get.add("null")))
     else if (nullable && oneOf.isDefined)
-      copy(oneOf = Some(oneOf.get :+ typeNull))
+      copy(oneOf = Some((oneOf.get :+ typeNull).distinct))
     else if (nullable && allOf.isDefined)
-      SerializableJsonSchema(allOf = Some(Chunk(this, typeNull)))
+      SerializableJsonSchema(allOf = Some((allOf.get :+ typeNull).distinct))
     else if (nullable && anyOf.isDefined)
-      copy(anyOf = Some(anyOf.get :+ typeNull))
+      copy(anyOf = Some((anyOf.get :+ typeNull).distinct))
     else if (nullable && ref.isDefined)
       SerializableJsonSchema(anyOf = Some(Chunk(typeNull, this)))
     else

zio-http/shared/src/main/scala/zio/http/endpoint/openapi/OpenAPIGen.scala

@@ -984,6 +984,10 @@ object OpenAPIGen {
         }
       case t: Transform[_, _, _]      =>
         nominal(t.schema, referenceType)
+      case Schema.Optional(inner, _)  =>
+        nominal(inner, referenceType)
+      case Schema.Lazy(schema0)       =>
+        nominal(schema0(), referenceType)
       case _                          => None
     }