Don't abort on an empty static aliases file

An empty static alias file specified via, e.g.,

    table aliases file:/etc/aliases

will cause smtpd to abort with

    invalid use of table "aliases" as ALIAS parameter.

This is because it doesn't get assigned the T_DYNAMIC flag by virtue of using
the static backend. Moreover, since it contains no lines, the backend is unable
to divine whether the file should be treated as a T_HASH or a T_LIST, and so
neither of these flags. Thus, when smtpd checks to see that the « aliases »
table satisfies T_DYNAMIC|T_HASH (neither of which are satisfied by the empty
aliases file) at startup, the check fails and smtpd aborts.