Handle undef values in magic.t on VMS.

888a67f6b95603701 added a new test that failed on VMS because it
expected the value of an undefined element in %ENV to be an empty
string, whereas on VMS it's actually a one-byte string containing
a null.  This is made possible by the fact that the logical name
API uses strings of known lengths, and made necessary by the fact
that a zero-length value (equivalence name in VMS parlance) is
not allowed.

So patch up the test implementation to handle this special case.