hw/riscv: Don't add empty bootargs to device tree

Commit 7c28f4da ("RISC-V: Don't add NULL bootargs to device-tree")
tried to avoid adding *NULL* bootargs to device tree, but unfortunately
the changes were entirely useless, due to MachineState::kernel_cmdline
can't be NULL at all as the default value is given as an empty string.
(see hw/core/machine.c::machine_initfn()).

Note the wording of *NULL* bootargs is wrong. It can't be NULL otherwise
a segfault had already been observed by dereferencing the NULL pointer.
It should be worded as *empty" bootargs.

Fixes: 7c28f4da ("RISC-V: Don't add NULL bootargs to device-tree")
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220421055629.1177285-2-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>