nvme/056: Add check for empty ddp capabilities

[nuclearcat]

Fixes a shell syntax error that occurs when trying to perform bitwise AND operation on empty ddp capabilities.
Now explicitly checks if ddp_caps hw returns empty string before proceeding with capability check.
Improves error handling for non-ddp capable devices like qemu virtio-net.

Without this patch qemu with virtio-net returns error:

tests/nvme/056: line 222: & 3: syntax error: operand expected (error token is "& 3")

With patch:

nvme/056 (tr=tcp) (enable zero copy offload and run rw traffic) [not run]
    runtime    ...  0.150s
    No ddp capabilities found for enp0s6

[kawasaki]

@nuclearcat Thanks for the fix. It looks good to me. I'll wait a few more days before merge just in case @aaptel has any comments.

@aaptel FYI.

[kawasaki]

I have merged this PR. Thanks!