Fix schema serialization with empty enumerations

This is an obvious oversight from me when adding tests in the original
PR. I managed to test array schema evolution for extending enumerations
but never thought to add basic schema serialization tests with empty
enmerations.

test/src/unit-enumerations.cc

@@ -2119,6 +2119,26 @@ TEST_CASE_METHOD(
   REQUIRE(loaded_names2.empty());
 }
 
+TEST_CASE_METHOD(
+    EnumerationFx,
+    "Cap'N Proto - ArraySchema Serialization with Empty Enumerations",
+    "[enumeration][capnp][array-schema][empty-enumerations]") {
+  create_array();
+
+  auto client_side = GENERATE(true, false);
+  auto ser_type = GENERATE(SerializationType::CAPNP, SerializationType::JSON);
+
+  auto schema1 = create_schema();
+  auto empty_enmr = Enumeration::create("empty", Datatype::INT32, 1, false, nullptr, 0, nullptr, 0);
+  schema1->add_enumeration(empty_enmr);
+
+  auto schema2 = ser_des_array_schema(schema1, client_side, ser_type);
+
+  auto all_names1 = schema1->get_enumeration_names();
+  auto all_names2 = schema2.get_enumeration_names();
+  REQUIRE(vec_cmp(all_names1, all_names2));
+}
+
 TEST_CASE_METHOD(
     EnumerationFx,
     "Cap'N Proto - Basic ArraySchemaEvolution Serialization",

tiledb/sm/serialization/enumeration.cc

@@ -58,7 +58,9 @@ void enumeration_to_capnp(
   enmr_builder.setOrdered(enumeration->ordered());
 
   auto dspan = enumeration->data();
-  enmr_builder.setData(::kj::arrayPtr(dspan.data(), dspan.size()));
+  if (dspan.size() > 0) {
+    enmr_builder.setData(::kj::arrayPtr(dspan.data(), dspan.size()));
+  }
 
   if (enumeration->var_size()) {
     auto ospan = enumeration->offsets();
@@ -73,15 +75,14 @@ shared_ptr<const Enumeration> enumeration_from_capnp(
   Datatype datatype = Datatype::ANY;
   throw_if_not_ok(datatype_enum(reader.getType(), &datatype));
 
-  if (!reader.hasData()) {
-    throw SerializationStatusException(
-        "[Deserialization::enumeration_from_capnp] Deserialization of "
-        "Enumeration is missing its data buffer.");
-  }
+  const void* data = nullptr;
+  uint64_t data_size = 0;
 
-  auto data_reader = reader.getData().asBytes();
-  auto data = data_reader.begin();
-  auto data_size = data_reader.size();
+  if (reader.hasData()) {
+    auto data_reader = reader.getData().asBytes();
+    data = data_reader.begin();
+    data_size = data_reader.size();
+  }
 
   const void* offsets = nullptr;
   uint64_t offsets_size = 0;