Add missing 'None' option for category

Signed-off-by: Pekka Helenius <fincer89@hotmail.com>

bookstore/src/main/resources/templates/bookadd.html

@@ -169,6 +169,7 @@
     				th:value="${category.id}"
     				th:text="${category.name}"
     				>(obj) category.name</option>
+    				<option value="">None</option>
     			</select>
     			<small class="form-text text-muted"
     			th:text="${#messages.msgOrNull('book.desc.set.category')} ?: 'book.desc.set.category'">