efi/printf: Handle null string input

Print "(null)" for 's' if the input is a NULL pointer.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200518190716.751506-14-nivedita@alum.mit.edu


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>