Short-circuit out of install with no packages (#569)
Short-circuit out of install with no packages (#569)

The degenerate case of `NpmUtilities.installInDir` is an install of no
packages.  The _behavior_ in this case was previously to run an `npm install`
with no arguments, which installs _everything_.

This patch short-circuits out of the noop.
FaHeymann/lerna,sebmck/lerna,lerna/lerna,onigoetz/lerna-hg,tbayaa/lerna,lerna/lerna,kittens/lerna,evocateur/lerna,treshugart/lerna,lerna/lerna,evocateur/lerna