Fixed crash in filename discovery when defining a stylesheet with raw content.

Author: mhaylock

lib/swift/playground/assets/stylesheet.rb

@@ -18,7 +18,7 @@ def save(destination_path, number)
 
       def derived_filename(pathname_or_content)
         filename = super(pathname_or_content)
-        filename.gsub(/\.scss$/, '')
+        filename.gsub(/\.scss$/, '') if filename
       end
 
       private