Add support for passing empty strings from Kotlin to Rust

[emarteca]

Previously, we couldn't pass empty strings from Kotlin to Rust. This is because string passing from Kotlin to Rust involves the allocation of a JNA Memory object, which can't be creating with a size of zero, so passing an empty string results in a runtime exception.

In this PR, we add support for passing empty strings from Kotlin to Rust. This is done by casing out Memory allocation so that if the allocated size is zero we actually allocate a size of 1, and then clear it.

Fixes #749

[Manishearth]

Hold on, for an empty string can we not just pass down a pointer of value 0x1?

[emarteca]

Oh hmm, that's true. To do that, I think we'd have to overhaul the current system to have the native (and a few of the read) functions return optional Memory, where it returns null if the length of the slice passed in is zero. And then only close the Memory if it's non-null.
That might be cleaner overall, what do you think?

[Manishearth]

Yes, that makes sense.

[emarteca]

Done! Good idea, it's much nicer this way 🙏