Fix potential undefined index on request submission

When submitting a request for an account without a `HTTP_USER_AGENT`, a
`NOTICE` is emitted:

    Notice: Undefined index: HTTP_USER_AGENT in /var/www/includes/request.php on line 634

This patch changes the submit function to check for existence of the
`HTTP_USER_AGENT` header before adding it to the database.
Fixup pullreq

We want to set the useragent field in the database to a value which is
clearly unset - not a string value of "NULL".
The section above is done differently to the implementation suggested.
Merge branch 'koentje_fix_undefined_index'