Use explicit empty spans rather than null or empty arrays (#604)

Author: adamreeve

csharp.test/TestColumnWriter.cs

@@ -21,7 +21,7 @@ public static void TestWriteBatchWithNullOptionalField()
                 var defLevels = new short[] { 1, 0, 1 };
                 var values = new[] { 1, 2 };
 
-                colWriter.WriteBatch(defLevels.Length, defLevels, null, values);
+                colWriter.WriteBatch(defLevels.Length, defLevels, ReadOnlySpan<short>.Empty, values);
 
                 writer.Close();
             }

csharp/ColumnReader.cs

@@ -222,7 +222,7 @@ public override TReturn Apply<TReturn>(IColumnReaderVisitor<TReturn> visitor)
 
         public long ReadBatch(long batchSize, Span<TValue> values, out long valuesRead)
         {
-            return ReadBatch(batchSize, null, null, values, out valuesRead);
+            return ReadBatch(batchSize, Span<short>.Empty, Span<short>.Empty, values, out valuesRead);
         }
 
         public unsafe long ReadBatch(long batchSize, Span<short> defLevels, Span<short> repLevels, Span<TValue> values, out long valuesRead)

csharp/ColumnWriter.cs

@@ -271,7 +271,7 @@ public override TReturn Apply<TReturn>(IColumnWriterVisitor<TReturn> visitor)
         /// <param name="values">The values to write.</param>
         public void WriteBatch(ReadOnlySpan<TValue> values)
         {
-            WriteBatch(values.Length, null, null, values);
+            WriteBatch(values.Length, ReadOnlySpan<short>.Empty, ReadOnlySpan<short>.Empty, values);
         }
 
         /// <summary>

csharp/LogicalBatchWriter/ArrayWriter.cs

@@ -49,7 +49,7 @@ public void WriteBatch(ReadOnlySpan<TItem[]> values)
                     {
                         // Write zero length array
                         _physicalWriter.WriteBatch(
-                            1, arrayDefinitionLevel, arrayRepetitionLevel, Array.Empty<TPhysical>());
+                            1, arrayDefinitionLevel, arrayRepetitionLevel, ReadOnlySpan<TPhysical>.Empty);
                     }
                 }
                 else if (!_optionalArrays)
@@ -60,7 +60,7 @@ public void WriteBatch(ReadOnlySpan<TItem[]> values)
                 {
                     // Write a null array entry
                     _physicalWriter.WriteBatch(
-                        1, nullDefinitionLevel, arrayRepetitionLevel, Array.Empty<TPhysical>());
+                        1, nullDefinitionLevel, arrayRepetitionLevel, ReadOnlySpan<TPhysical>.Empty);
                 }
 
                 if (i == 0)

csharp/LogicalBatchWriter/OptionalNestedWriter.cs

@@ -98,8 +98,8 @@ public void WriteBatch(ReadOnlySpan<Nested<TItem>?> values)
                     _physicalWriter.WriteBatch(
                         nullSpanSize,
                         _buffers.DefLevels.AsSpan(0, nullSpanSize),
-                        _buffers.RepLevels == null ? null : _buffers.RepLevels.AsSpan(0, nullSpanSize),
-                        Array.Empty<TPhysical>());
+                        _buffers.RepLevels == null ? ReadOnlySpan<short>.Empty : _buffers.RepLevels.AsSpan(0, nullSpanSize),
+                        ReadOnlySpan<TPhysical>.Empty);
                     offset += nullSpanSize;
                 }