More tests and NPE-proofing

Author: garydgregory

src/main/java/org/apache/commons/io/file/PathUtils.java

@@ -878,7 +878,7 @@ public static DosFileAttributeView getDosFileAttributeView(final Path path, fina
      * @since 2.16.0
      */
     public static <R> R getFileName(final Path path, Function<Path, R> function) {
-        final Path fileName = path.getFileName();
+        final Path fileName = path != null ? path.getFileName() : null;
         return fileName != null ? function.apply(fileName) : null;
     }
 

src/test/java/org/apache/commons/io/file/PathUtilsTest.java

@@ -246,6 +246,17 @@ public void testCreateDirectoriesSymlinkClashing() throws IOException {
         assertEquals(symlinkedDir, PathUtils.createParentDirectories(symlinkedDir.resolve("child")));
     }
 
+    @Test
+    public void testGetFileName() {
+        assertNull(PathUtils.getFileName(null, null));
+        assertNull(PathUtils.getFileName(null, Path::toString));
+        assertNull(PathUtils.getFileName(Paths.get("/"), Path::toString));
+        assertNull(PathUtils.getFileName(Paths.get("/"), Path::toString));
+        assertEquals("", PathUtils.getFileName(Paths.get(""), Path::toString));
+        assertEquals("a", PathUtils.getFileName(Paths.get("a"), Path::toString));
+        assertEquals("a", PathUtils.getFileName(Paths.get("p", "a"), Path::toString));
+    }
+
     @Test
     public void testGetFileNameString() {
         assertNull(PathUtils.getFileNameString(Paths.get("/")));