mtd: mtdpart: Do not supply NULL to printf()

GCC compiler is not happy about NULL being supplied as printf() parameter:

drivers/mtd/mtdpart.c:693:34: error: ‘%s’ directive argument is null [-Werror=format-overflow=]

Move the code after the parser test for NULL, and drop the ternary completely.
The user can deduct this since when it's not NULL two messages will be printed.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>