Fix false negative test failure in ResourceTests

Prior to changes in commit 57851de,
AbstractResource#getFilename threw IllegalStateException unless
overridden by a subclass. Following that change, this method now throws
null instead, but ResourceTests#testAbstractResourceExceptions had not
been updated to reflect, resulting in a false negative failure. This has
now been fixed.

Issue: SPR-9043

org.springframework.core/src/test/java/org/springframework/core/io/ResourceTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.core.io;
 
+import static org.hamcrest.CoreMatchers.nullValue;
 import static org.junit.Assert.*;
 
 import java.io.ByteArrayInputStream;
@@ -215,13 +216,8 @@ public InputStream getInputStream() {
 		catch (FileNotFoundException ex) {
 			assertTrue(ex.getMessage().indexOf(name) != -1);
 		}
-		try {
-			resource.getFilename();
-			fail("IllegalStateException should have been thrown");
-		}
-		catch (IllegalStateException ex) {
-			assertTrue(ex.getMessage().indexOf(name) != -1);
-		}
+
+		assertThat(resource.getFilename(), nullValue());
 	}
 
 }