DATACMNS-387 - Improvements in null handling in PartTree area.

We're now rejecting invalid constructor arguments handed to ClassTypeInformation, Part, PartTree and PropertyPath. Beyond that we skip the creation of a Part for an empty path segment, so that you don't end up with an invalid Part instance for a findAllByOrderByFooAsc.

src/test/java/org/springframework/data/mapping/PropertyPathUnitTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2011-2012 the original author or authors.
+ * Copyright 2011-2013 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@ package org.springframework.data.mapping;
 
 import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.*;
+import static org.springframework.data.mapping.PropertyPath.*;
 
 import java.util.Iterator;
 import java.util.Map;
@@ -319,6 +320,38 @@ public class PropertyPathUnitTests {
 		PropertyPath.from("userNameBar", Bar.class);
 	}
 
+	/**
+	 * @see DATACMNS-387
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void rejectsNullSource() {
+		from(null, Foo.class);
+	}
+
+	/**
+	 * @see DATACMNS-387
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void rejectsEmptySource() {
+		from("", Foo.class);
+	}
+
+	/**
+	 * @see DATACMNS-387
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void rejectsNullClass() {
+		from("foo", (Class<?>) null);
+	}
+
+	/**
+	 * @see DATACMNS-387
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void rejectsNullTypeInformation() {
+		from("foo", (TypeInformation<?>) null);
+	}
+
 	private class Foo {
 
 		String userName;