BUG: Fix windows command line escape for empty arg
BUG: Fix windows command line escape for empty arg

On Windows the KWSys System package generates escapes for command-line
arguments.  This fix enables quoting of the empty string as an argument.
This also adds a test to pass an empty argument to a custom command.

chuckatkins/KWSys,chuckatkins/KWSys,chuckatkins/KWSys,chuckatkins/KWSys