Simplify App#run by using Null Object pattern.

Welcome, Guides, Videos, Examples and Categories now all have a
static #create method to be used instead of #new for instantiation.

The #create returns either the corresponding object instance or
NullObject which behaves as if it were the needed object, but
effectively does nothing.

Categories.create is a bit different - it first uses one of two
strategies for creating categories: FileCategories when filename
present or AutoCategories when not, then feeding the resulting
categories list to actual Categories object.