1. If empty, the variable has no effect. This is not a secure setting.
2. If set to the name of a directory, the server limits import and export operations to work only with files in that directory. The directory must exist; the server will not create it.
3. If set to NULL, the server disables import and export operations. This value is permitted as of MySQL 5.7.6.
Видимо правильным будет задать ее как secure-file-priv="" или указать на какой-либо существующий каталог