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.