Add unit tests for constructor null arguments on MessageExtras.

Author: Quintin Willison

lib/src/test/java/io/ably/lib/types/MessageExtrasTest.java

@@ -58,6 +58,11 @@ public void rawViaMessagePack() throws IOException {
 		assertEquals(messageExtras, unpacked);
 	}
 
+	@Test(expected = NullPointerException.class)
+	public void rawNullArgument() {
+		new MessageExtras((JsonObject)null);
+	}
+
 	/**
 	 * Construct an instance with DeltaExtras and validate that the
 	 * serialised JSON is as expected. Also validate that the DeltaExtras
@@ -108,6 +113,11 @@ public void deltaViaMessagePack() throws IOException {
 		assertEquals(expectedMessageExtrasJsonObject, unpacked.asJsonObject());
 	}
 
+	@Test(expected = NullPointerException.class)
+	public void deltaNullArgument() {
+		new MessageExtras((DeltaExtras)null);
+	}
+
 	private static JsonObject deltaExtrasJsonObject(final String format, final String from) {
 		final JsonObject deltaExtrasJsonElement = new JsonObject();
 		deltaExtrasJsonElement.addProperty("format", format);