env and isdir macro updates/fixes

env macro should return "" when environment not found. Before this fix, it would return ${env;BLAR}, which would actually break an ${if} macro.

Tweak _isdir macro to allow no parameters. No parameters means false (e.g.
no directory).

Signed-off-by: Carter Smithhart <carter.smithhart@gmail.com>
Merge pull request #733 from derheld42/macrofix

env and isdir macro updates/fixes