Fixed splitting on nil error.

    ~$ irb
    load error: /home/user/.rvm/rubies/ruby-2.1.2/.irbrc
    NoMethodError: undefined method `split' for nil:NilClass
    	/home/user/.rvm/scripts/irbrc.rb:45:in `<top (required)>'
    	/home/user/.rvm/rubies/ruby-2.1.2/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    	/home/user/.rvm/rubies/ruby-2.1.2/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    	/home/user/.rvm/rubies/ruby-2.1.2/.irbrc:11:in `<top (required)>'
    	/home/user/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/irb/init.rb:236:in `load'
    	/home/user/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/irb/init.rb:236:in `run_config'
    	/home/user/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/irb/init.rb:19:in `setup'