Correctly handle empty STOW_DIR (#5, #6)

In shell, a variable is often considered unset even if it is
set to the empty string.  In other words,

  STOW_DIR= stow [args]

is an idiomatic alternative to writing:

  unset STOW_DIR
  stow [args]

and it also has the advantage of temporarily "unsetting" STOW_DIR
for a single command.

Therefore we should treat STOW_DIR being set to the empty string
as equivalent to it not being set at all.

Thanks to Cuong Manh Le for highlighting this issue!

Fixes #6  - https://github.com/aspiers/stow/issues/6
Closes #5 - https://github.com/aspiers/stow/pull/5

THANKS

@@ -23,3 +23,4 @@ Christopher Hoobin  <christopher.hoobin.edu.au>
 Adam Spiers         <stow@adamspiers.org>
 Stefano Lattarini
 Adam Sampson
+Cuong Manh Le