test: do no assume hush parser in validate_empty()

The environment variable test uses function validate_empty() to check that
a variable is not defined. If the hush parser is not enabled, we cannot
refer to a variable by $var_name but only by ${var_name}.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Stephen Warren <swarren@nvidia.com>