modify work place in null but can create course

app/views/layouts/base_courses.html.erb

@@ -78,7 +78,7 @@
 											<tr>
 												<td valign="top" style="padding-left: 8px;">课程学期：</td><td class="font_lighter_sidebar"><%= @course.term %></td>
 											</tr>
-											<% unless @course.teacher.user_extensions.occupation==""%>											
+											<% unless @course.teacher.user_extensions.nil?%>											
 											<tr>
 												<td valign="top" style="padding-left: 8px;">教师单位：</td><td class="font_lighter_sidebar"><%= @course.teacher.user_extensions.occupation %></td>
 											</tr>