Also consider NULL/None category type for books

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

bookstore/src/main/resources/templates/bookedit.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"

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

@@ -37,7 +37,7 @@
 				<td th:text="${book.title}">(obj) book.title</td>
 				<td th:text="${book.isbn}">(obj) book.isbn</td>
 				<td th:text="${book.year}">(obj) book.year</td>
-				<td th:text="${book.category.name}">(obj) book.category.name</td>
+				<td th:text="${book.category != null} ? ${book.category.name} : 'None'">(obj) book.category.name</td>
 				<td th:text="(${#numbers.formatDecimal(book.price, 1, 2)} ?: '0.00') + ' ' + (${#messages.msgOrNull('page.symbols.currency')} ?: 'page.symbols.currency')">
 				0.00 page.symbols.currency
 				</td>