changelog.php: fix if version table is empty
changelog.php: fix if version table is empty

If the `version` table is empty, then this page doesn't append to
`$PHP_OUTPUT` and therefore gives a blank page (with a PHP warning).

In this case, we simply add a line saying that an admin must
add an initial version to the database first.

hemberger/smr,hemberger/smr,smrealms/smr,hemberger/smr,smrealms/smr,hemberger/smr,smrealms/smr