THRIFT-5439: Lua Generator: not support const i64

[hkb1990]

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[Jens-G]

Some more information about the problem and why the patch solves it would be super awesome.

[hkb1990]

Some more information about the problem and why the patch solves it would be super awesome.

described in jira ticket 5439.
const i64 is not generated correctly to lua. it is always generated to lualongnumber.new('') , no matter what value it is. Because t_base_type is i64, get_string returns empty.