Smaller reproduction for https://dev.fennel-lang.org/ticket/82.

Sample output

cargo run example.lua:

[src/main.rs:17:5] &lua_value = {
  example = "Not null",
  observations = {
    data = {
      {
        null_value = null,
        present = 1,
      },
    },
  },
}
{
  example = 'Not null',
  observations = {
    data = { {
        present = 1
      } }
  }
}
nil
----
1	table: 0x5dca4c772440
----

thread 'main' (2186) panicked at src/main.rs:29:41:
called `Result::unwrap()` on an `Err` value: RuntimeError("./fennel.lua:5302: attempt to index a boolean value (local '_69_0')\nstack traceback:\n\t[C]: in metamethod 'index'\n\t./fennel.lua:5302: in function <./fennel.lua:5292>\n\t(...tail calls...)\n\t./fennel.lua:5156: in upvalue 'pp_associative'\n\t./fennel.lua:5327: in function <./fennel.lua:5292>\n\t(...tail calls...)\n\t./fennel.lua:5156: in upvalue 'pp_associative'\n\t./fennel.lua:5327: in function <./fennel.lua:5292>\n\t(...tail calls...)\n\texample.fnl:2: in main chunk\n\t(...tail calls...)\n\texample.lua:17: in main chunk")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace