Parse last_modified time only if it is valid

fixes fog#426

Author: krishnasrinivas

lib/fog/azurerm/utilities/general.rb

@@ -156,8 +156,8 @@ def parse_storage_object(object)
       end
     end
   end
-
-  data['last_modified'] = Time.parse(data['last_modified'])
+  last_modified = data['last_modified']
+  data['last_modified'] = Time.parse(last_modified) if last_modified
   data['etag'].delete!('"')
   data
 end