Don't print "overrides project version (null)"

Specifically, when using checkout meteor with a project with no
.meteor/release file (ie, a pre-0.6.0 app, not a "none"-pinned app), we
used to print

  => Running Meteor from a checkout -- overrides project version (null)

but now we won't.
Avoid stack trace with empty '.meteor/release' file

This is not a regression against 0.7.0.1.

Fixes #1849.