revdep `collapse` fails `data.table`-related tests

[aitap]

Unfortunate result of #7451. Confirmed during CRAN checks:

    ══ Failed tests ════════════════════════════════════════════════════════════════
    ── Error ('test-data.table.R:216:3'): creating columns and printing works after passing a data.table through collapse functions ──
    Error in ``[.data.table`(dt, , `:=`(new, 1))`: Internal error in assign: selfrefnames is ok but maxLength(names) [12] != maxLength(dt) [1036]. Please report to the data.table issues tracker.
    Backtrace:
         ▆
      1. ├─testthat::expect_silent(dt[, `:=`(new, 1)]) at test-data.table.R:216:3
      2. │ └─testthat:::quasi_capture(enquo(object), NULL, evaluate_promise)
      3. │   ├─testthat (local) .capture(...)
      4. │   │ ├─withr::with_output_sink(...)
      5. │   │ │ └─base::force(code)
      6. │   │ ├─base::withCallingHandlers(...)
      7. │   │ └─base::withVisible(code)
      8. │   └─rlang::eval_bare(quo_get_expr(.quo), quo_get_env(.quo))
      9. ├─dt[, `:=`(new, 1)]
     10. └─data.table:::`[.data.table`(dt, , `:=`(new, 1))
    
    [ FAIL 1 | WARN 0 | SKIP 2 | PASS 17726 ]

filed fastverse/collapse#809, tested with both 1.17.8 and to-be-1.18.0.