Merge branch 'sa/git-var-empty'

"git var UNKNOWN_VARIABLE" and "git var VARIABLE" with the variable
given an empty value used to behave identically.  Now the latter
just gives an empty output, while the former still gives an error
message.

* sa/git-var-empty:
  var: allow GIT_EDITOR to return null
  var: do not print usage() with a correct invocation