server: don't call method for empty command array

This error would occur when pressing ENTER in tork-remote tork-driver:

tork-driver: #<ArgumentError: no method name given>
tork-driver: /home/skurapati/lab/tork/lib/tork/server.rb:74:in `recv'
tork-driver: /home/skurapati/lab/tork/lib/tork/driver.rb:64:in `recv'
tork-driver: /home/skurapati/lab/tork/lib/tork/server.rb:46:in `block (2 levels) in loop'
tork-driver: /home/skurapati/lab/tork/lib/tork/server.rb:37:in `each'
tork-driver: /home/skurapati/lab/tork/lib/tork/server.rb:37:in `block in loop'
tork-driver: /home/skurapati/lab/tork/lib/tork/server.rb:35:in `catch'
tork-driver: /home/skurapati/lab/tork/lib/tork/server.rb:35:in `loop'
tork-driver: /home/skurapati/lab/tork/lib/tork/driver.rb:21:in `loop'
tork-driver: /home/skurapati/lab/tork/bin/tork-driver:173:in `<top (required)>'
tork-driver: /home/skurapati/.rvm/gems/ruby-1.9.3-p448/bin/tork-driver:23:in `load'
tork-driver: /home/skurapati/.rvm/gems/ruby-1.9.3-p448/bin/tork-driver:23:in `<main>'