Add empty string concat test.
Add empty string concat test.

bnoordhuis/node-buffertools,chrisdew/node-buffertools,bnoordhuis/node-buffertools,chrisdew/node-buffertools,bnoordhuis/node-buffertools,chrisdew/node-buffertools,bnoordhuis/node-buffertools,chrisdew/node-buffertools