assemble: Remove stale `null` statement
assemble: Remove stale `null` statement

Because FFI expands to expression, even though it might be
a statement i.e. assignment, there are cases where these
statements will try to mimic like expression by giving a null
value. These are effectively NOP and we remove them.

vishesh/racketscript,vishesh/racketscript,vishesh/racketscript