Fix deriving JsonSchema on enum without variants

Fixes #287

schemars/tests/enum.rs

@@ -136,3 +136,11 @@ enum SoundOfMusic {
 fn enum_unit_with_doc_comments() -> TestResult {
     test_default_generated_schema::<SoundOfMusic>("enum-unit-doc")
 }
+
+#[derive(JsonSchema)]
+enum NoVariants {}
+
+#[test]
+fn enum_no_variants() -> TestResult {
+    test_default_generated_schema::<NoVariants>("no-variants")
+}

schemars/tests/expected/no-variants.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "NoVariants",
+  "type": "string",
+  "enum": []
+}