title: Invalid <url-pattern> in servlet mapping when set it to empty string
body: <p>To have a home-page servlet which will generate home page's content, I kept url-pattern empty in the servlet-mapping, which AFAIK maps to the context root, say like mysite.com/ or http://{host}:{port}/mysite/</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_0.xsd"
       version="3.0"&gt;
  
  &lt;display-name&gt;My site&lt;/display-name&gt;
  
  &lt;servlet&gt;
    &lt;servlet-name&gt;controller&lt;/servlet-name&gt;
    &lt;servlet-class&gt;com.mysite.control.MainController&lt;/servlet-class&gt;
    &lt;!-- Load this servlet at server startup time --&gt;
    &lt;load-on-startup&gt;1&lt;/load-on-startup&gt;
  &lt;/servlet&gt;
  
  &lt;servlet-mapping&gt;
    &lt;servlet-name&gt;controller&lt;/servlet-name&gt;
    &lt;url-pattern&gt;&lt;/url-pattern&gt;
  &lt;/servlet-mapping&gt;  
  </code></pre>
  
  <p></p>
  
  <p>The above is the complete web.xml (except root element web-app).
  But Tomcat 7.0.26 gives following error during startup due to which the war is not getting deployed.</p>
  
  <pre><code>SEVERE: ContainerBase.addChild: start: 
  org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext[/mysite]]
      at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:154)
      at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:895)
      at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:871)
      at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:615)
      at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:958)
      ...
  Caused by: java.lang.IllegalArgumentException: Invalid &lt;url-pattern&gt;  in servlet mapping
      at org.apache.catalina.core.StandardContext.addServletMapping(StandardContext.java:3208)
  ...
  </code></pre>
  
  <p><strong>Note 1:</strong> This is not a spring project, it is pure servlet 3.0 project and just has web.xml configuration at the moment.</p>
  
  <p><strong>Note 2:</strong> The class com.mysite.control.MainController does exist.</p>
  
  <p>How can I fix this? I know welcome-file is an option to mimic home page but that is not optimal and I would prefer to fix this instead. Is this tomcat 7.0.26 bug or some other issue?</p>