When updating an export template, the exportType request parameter is
ignored by the controller, because it's not used by the UPDATE
statement. However the Template value object requires a valid ExportType
enum, causing a ValueError to be thrown.
```
ValueError: "" is not a valid backing value for enum PhpMyAdmin\Plugins\ExportType
```
Signed-off-by: Maurício Meneghini Fauth <mauricio@mfauth.net>