Add test for empty RNTuple

Closes #33

Author: hahnjo

structure/README.md

@@ -5,3 +5,4 @@ Therefore all tests in this category write a single `Int32` field with type `std
 
  * [`clusters`](clusters): multiple clusters
  * [`cluster_groups`](cluster_groups): multiple cluster groups
+ * [`empty`](empty): empty RNTuple without entries

structure/empty/README.md

@@ -0,0 +1,3 @@
+# Empty RNTuple
+
+An RNTuple with a field, but no entry.

structure/empty/read.C

@@ -0,0 +1,6 @@
+#include "../read_structure.hxx"
+
+void read(std::string_view input = "structure.empty.root",
+          std::string_view output = "structure.empty.json") {
+  read_structure(input, output);
+}

structure/empty/write.C

@@ -0,0 +1,24 @@
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+using ROOT::Experimental::RNTupleModel;
+using ROOT::Experimental::RNTupleWriteOptions;
+using ROOT::Experimental::RNTupleWriter;
+
+#include <cstdint>
+#include <string_view>
+
+void write(std::string_view filename = "structure.empty.root") {
+  auto model = RNTupleModel::Create();
+
+  auto Int32 = model->MakeField<std::int32_t>("Int32");
+
+  RNTupleWriteOptions options;
+  options.SetCompression(0);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  // Destruct the writer / commit the dataset without filling an entry.
+  writer.reset();
+}

structure/read_structure.hxx

@@ -31,6 +31,8 @@ void read_structure(std::string_view input, std::string_view output) {
     // Newline is intentionally missing, may need to print a comma before the
     // next entry.
   }
-  os << "\n";
+  if (!first) {
+    os << "\n";
+  }
   os << "]\n";
 }