Fix panic on sequence_init() when size is 0

[Oscarchoi]

resolves: #406

In Rust 1.78, a panic occurs when publishing sequence-type messages containing array types, which could be fixed by adding null pointer handling.

[jhdcs]

The change appears to add an appropriate guard to prevent a panic. I'm not seeing any immediate issues from the change, since the code looks like it otherwise can handle a null pointer, but a second reviewer would be appreciated.

[esteve]

@Oscarchoi LGTM, thanks!