Resolve #3201566 "Notice trying to access array offset of type null"

Closes #3201566 : Fix the PHP Notice trying to access array offset on value of type null in _cdn_build_css_path()

README.txt

@@ -91,10 +91,12 @@ Installation
    The CDN will provide you with a "delivery address", this is the address
    that we'll use to download files from the CDN instead of the Drupal server.
    Suppose this is `http://d85nwn7m5gl3y.cloudfront.net`.
+   Be sure to forward query strings from the CDN to the origin! Otherwise image
+   style derivatives will not work.
    (It acts like a globally distributed, super fast proxy server.)
 
    Relevant links:
-   - Amazon CloudFront: http://docs.amazonwebservices.com/AmazonCloudFront/latest/DeveloperGuide/CreatingDistributions.html?r=4212
+   - Amazon CloudFront gotcha: https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/QueryStringParameters.html
 
 5) Optionally, you can create a CNAME alias to the delivery address on your
    DNS server. This way, it's not immediately obvious from the links in the
@@ -115,6 +117,14 @@ Installation
    your site with your root/admin (user id 1) account. The statistics will
    show which files are served from the CDN!
 
+9) If your site is behind a reverse proxy such as Varnish, so that your stack
+   looks like: CDN <-> reverse proxy <-> web server, then you need to take extra
+   measures if you want to prevent duplicate content showing up on the CDN. See
+   https://www.drupal.org/node/2678374#comment-11278951 for details. It's
+   possible in this situation to end up with redirect loops; for that reason
+   the CDN module adds a debugging header to the 301 redirects it emits in order
+   to facilitate troubleshooting.
+
 
 File Conveyor mode
 ------------------
@@ -189,6 +199,9 @@ A: Nothing. It only serves as a sample for using File Conveyor. It's used for
 Q: How to use different CDNs based on the domain name of an i18n site?
 A: See http://drupal.org/node/1483962#comment-5744830.
 
+Q: Why are old CDN Far Future URLs not working?
+A: Your Drupal site's private key or hash salt have changed. See
+   https://www.drupal.org/node/1844786#comment-6832244 for details.
 
 No cookies should be sent to the CDN
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