Non-null value printed as [null] in assert_equals

[NickVolynkin]

reported by @imeevma

local server = require('test.luatest_helpers.server')
local t = require('luatest')
local g = t.group()

g.before_all(function()
    g.server = server:new({alias = 'repro'})
    g.server:start()
    g.server:exec(function()
        local s = box.schema.space.create('T', {format = {{'DEC', 'decimal'}}})
        s:create_index('I')
    end)
end)

g.after_all(function()
    g.server:exec(function()
        box.space.T:drop()
    end)
    g.server:stop()
end)

g.test_repro_1 = function()
    g.server:exec(function()
        local t = require('luatest')
        local dec = require('decimal')
        local s = box.space.T
        s:insert({dec.new(1)})
        -- must fail
        t.assert_equals(s:select(), {{dec.new(2)}})
    end)
end

output:

[001] sql-luatest/repro_test.lua                                      [ fail ]
[001] Test failed! Output from reject file var/rejects/sql-luatest/repro.reject:
[001] TAP version 13
[001] 1..1
[001] # Started on Thu Mar  3 12:45:05 2022
[001] # Starting group: sql-luatest.repro
[001] not ok 1    sql-luatest.repro.test_repro_1
[001] #   /home/mergen/work/dev/test/sql-luatest/repro_test.lua:27: expected: {{2}}
- [001] #   actual: {[null]}
+ [001] #   actual: {{1}}
[001] #   stack traceback:
[001] #       /home/mergen/work/dev/test/sql-luatest/repro_test.lua:22: in function 'sql-luatest.repro.test_repro_1'
[001] #       ...
[001] #       [C]: in function 'xpcall'
[001] # Ran 1 tests in 0.352 seconds, 0 succeeded, 1 failed