Don't generate an inline object for objects with no properties

The core doesn't really know how to emit objects like that (so it
doesn't, and we end up with a compile error when building the generated
classes).  At any rate, if there's a bare object type without any
properties, our assumption is that the user is expected to populate it
with arbitrary key/value pairs (sorta like a map, but I guess they get
more freedom for keys?), so in this case we should just use
objectType() (io.cire.Json or Jackson JsonNode).

modules/codegen/src/main/scala/com/twilio/guardrail/SwaggerUtil.scala

@@ -300,11 +300,13 @@ object SwaggerUtil {
       Sw: SwaggerTerms[L, F],
       Fw: FrameworkTerms[L, F]
   ): Free[F, ResolvedType[L]] = {
-    import Sc._
+    import Fw._
     val action: Free[F, ResolvedType[L]] = Free.pure(Resolved[L](tpe, None, None, None, None))
     propMetaImpl(property) {
-      case _: ObjectSchema =>
+      case schema: ObjectSchema if Option(schema.getProperties).exists(p => !p.isEmpty) =>
         action
+      case _: ObjectSchema =>
+        objectType(None).map(Resolved[L](_, None, None, None, None))
       case _: ComposedSchema =>
         action
     }

modules/sample/src/main/resources/additional-properties.yaml

@@ -5,6 +5,22 @@ info:
 paths: {}
 components:
   schemas:
+    SimpleMap:
+      type: object
+      required:
+        - simple_obj
+        - simple_str
+      properties:
+        simple_obj:
+          type: object
+          additionalProperties:
+            type: object
+        simple_str:
+          type: object
+          additionalProperties:
+            type: string
+        just_obj:
+          type: object
     FooMapValues:
       type: object
       required: