Empty string argument should be quoted

[njam]

Currently an empty string argument becomes an empty string, and is thus ignored by the shell:

$ node -e "let esc = require('shell-escape'); console.log('|' + esc(['']) + '|')"
||

Instead an empty string should become as '' I think.

[binki]

If I may suggest this, it is often better to try to use something like cross-spawn which accepts an array of arguments and knows how to target the current platform (see #9). You will have a more portable and safe program if you use that approach.