Fix #26 - Avoid notifying unassigned SharedArrayBuffer entry

[WebReflection]

This MR avoids a very subtle bug that is hard to repeat consistently but that, to my understanding, boils down to this issue:

• if the shared array buffer is not changed but notified as "nothing to return" without an explicit value set to its index [0] then Atomics.wait(sb, 0, 0, delay) might fail unexpectedly without any warning whatsoever
• if the shared array buffer has an explicit value set at index [0] then everything always works as expected
• if the remote function returns a value, hence it assign some length to the [0] it always works too

I have no idea if it's worth it to me to file a bug to all vendors as I think this is an intended behavior.

Changes

• be sure that if the remote call does not return anything (undefined) a value at index [0] is set as -1 so that negative result length means no further actions need to be performed
• when the length is indeed less than zero, early return without asking for data to unserialize