title: ModPagespeedFileCachePath must not be empty
body: <p>I'm running Debian and <a href="https://developers.google.com/speed/docs/mod_pagespeed/download" rel="nofollow">installed <strong>mod_pagespeed</strong></a> some weeks ago. Today there was a new version available when running apt-get update and upgrade. So I simply accepted it and ran it. </p>
  
  <p>After manually restarted my Apache, it says in the error.log:</p>
  
  <pre><code>[...] [error] [mod_pagespeed 1.3.25.4-2941 @12950] mod_pagespeed is enabled. ModPagespeedFileCachePath must not be empty: defn_name=(null) defn_line_number=0 server_hostname=mydomain.com port=0
  Configuration Failed
  </code></pre>
  
  <p>I deleted all stuff inside of <code>/var/cache/mod_pagespeed</code> (Which is the value of <code>ModPagespeedFileCachePath</code>) but it does not help. </p>
  
  <p>Also uninstalling and reinstalling mod_pagespeed does not help. </p>