Fix Kafka test which was broken by xUnit upgrade

The production code behavior hasn't changed here, but Assert.Equal used to pass when presented with an empty string and an empty byte array.

Signed-off-by: Jon Skeet <[email protected]>

test/CloudNative.CloudEvents.UnitTests/Kafka/KafkaTest.cs

@@ -132,7 +132,7 @@ public void KafkaBinaryMessageTest()
         [Theory]
         [InlineData(MediaTypeNames.Application.Octet, new byte[0])]
         [InlineData(MediaTypeNames.Application.Json, null)]
-        [InlineData(MediaTypeNames.Application.Xml, "")]
+        [InlineData(MediaTypeNames.Application.Xml, new byte[0])]
         [InlineData(MediaTypeNames.Text.Plain, "")]
         [InlineData(null, null)]
         public void KafkaBinaryMessageTombstoneTest(string? contentType, object? expectedDecodedResult)