This works because of the tiny lag gap caused by using a castsequence. Castsequence queries the server to see if the previous spell has completed casting before sending a message for the next one to cast. This allows the macro to slip through to the straight cast command in the second line. Nothing to do with the null as far as I can tell.
ie.
/castsequence reset=target lightning bolt, null
/cast lava burst
should work the same as
/castsequence reset=target lightning bolt, loghtning bolt
/cast lava burst.
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