Schema: use db_is_oracle() function
file_add(): Oracle BLOB contents must not be quoted

Using a quoted hex string causes error 'ORA-01465: invalid hex number
for the query' when inserting the new record.

Follow-up fix for issue #13227
Oracle: BLOB columns defaulted to empty_blob()

Fixes an issue preventing insertion of file attachments with
$g_file_upload_method = DISK causing error 'ORA-01400: cannot insert
NULL into ("MANTIS"."M_BUG_FILE_T"."CONTENT")'.

Since I can't do this "properly" without increasing the schema number
(which is not possible in 1.2.x branch), any users encountering this
issue should manually update their schema:

ALTER TABLE M_BUG_FILE_T MODIFY CONTENT DEFAULT empty_blob();
ALTER TABLE M_PROJECT_FILE_T MODIFY CONTENT DEFAULT empty_blob();

Fixes #16331