Fix warning from `get_formats_from_stats()`

[edelarua]

If any elements of levels_per_stats are unnamed and there is no default format, then a warning occurs.

Warning in integration tests for scda.test:

── Warning ('test-table_ext01.R:175:3'): EXT01 variant: with user specified categories for missed doses ──
  longer object length is not a multiple of shorter object length
  Backtrace:
       ▆
    1. └─rtables::build_table(lyt = lyt, df = anl, alt_counts_df = adsl) at test-table_ext01.R:175:3
    2.   └─base::lapply(...)
    3.     └─rtables (local) FUN(X[[i]], ...)
    4.       └─rtables:::recursive_applysplit(...)
    5.         ├─rtables:::.make_split_kids(...)
    6.         └─rtables:::.make_split_kids(...)
    7.           └─rtables (local) .local(spl, have_controws, make_lrow, ...)
    8.             ├─base::unlist(...)
    9.             └─base::mapply(...)
   10.               └─rtables (local) `<fn>`(...)
   11.                 └─rtables:::recursive_applysplit(...)
   12.                   ├─rtables:::.make_split_kids(...)
   13.                   └─rtables:::.make_split_kids(...)
   14.                     └─rtables (local) .local(spl, have_controws, make_lrow, ...)
   15.                       ├─base::unlist(...)
   16.                       └─base::lapply(...)
   17.                         ├─rtables (local) FUN(X[[i]], ...)
   18.                         └─rtables (local) FUN(X[[i]], ...)
   19.                           └─rtables (local) .local(spl, have_controws, make_lrow, ...)
   20.                             └─rtables:::.make_analyzed_tab(...)
   21.                               ├─base::tryCatch(...)
   22.                               │ └─base (local) tryCatchList(expr, classes, parentenv, handlers)
   23.                               │   └─base (local) tryCatchOne(expr, names, parentenv, handlers[[1L]])
   24.                               │     └─base (local) doTryCatch(return(expr), name, parentenv, handler)
   25.                               └─rtables:::.make_tablerows(...)
   26.                                 └─rtables:::gen_rowvalues(...)
   27.                                   └─base::mapply(...)
   28.                                     └─rtables (local) `<fn>`(...)
   29.                                       ├─base::do.call(func, args)
   30.                                       └─tern (local) `<fn>`(...)
   31.                                         └─tern::get_formats_from_stats(.stats, .formats, levels_per_stats)