Fix LuaStandalone nil handling

In Lua, a table entry with a nil value is the same as a table entry that
doesn't exist. So when serializing for transfer to PHP, these keys will
be skipped. For a table as an associative array this isn't much of a
problem, but for a table as a list it means we have missing indexes.
Some of Lua's functions for handling "lists" (i.e. tables with numeric
keys) also have a problem when the list contains nils.

To work around these issues when passing argument lists and return value
lists, pass the number of elements along with the sparse list. On the
PHP end we can use this to fill in the missing nulls, and on the Lua
end we can pass this count to unpack() to avoid the problems on the Lua
side.

Change-Id: I858e3905a06e377693301da2b8bc534808f00e3e