Allow NULL dimnames to propagate through new_table()

[DavisVaughan]

R-devel failures:

══ Failed tests ════════════════════════════════════════════════════════════════
     ── Failure ('test-type-table.R:21:3'): can find a common type among tables with identical dimensions ──
     `actual` (vec_ptype2(tab1, tab1)) not identical to `expected` (zap_dimnames(new_table())).
    
     `dimnames(actual)` is a list
     `dimnames(expected)` is absent
     Backtrace:
     ▆
     1. └─vctrs:::expect_identical(vec_ptype2(tab1, tab1), zap_dimnames(new_table())) at test-type-table.R:21:2
     2. └─vctrs:::expect_waldo_equal("identical", act, exp, info, ...) at tests/testthat/helper-vctrs.R:43:2
     ── Failure ('test-type-table.R:29:3'): size is not considered in the ptype ─────
     `actual` (vec_ptype2(x, y)) not identical to `expected` (zap_dimnames(new_table())).
    
     `dimnames(actual)` is a list
     `dimnames(expected)` is absent
     Backtrace:
     ▆
     1. └─vctrs:::expect_identical(vec_ptype2(x, y), zap_dimnames(new_table())) at test-type-table.R:29:2
     2. └─vctrs:::expect_waldo_equal("identical", act, exp, info, ...) at tests/testthat/helper-vctrs.R:43:2
     ── Failure ('test-type-table.R:81:3'): `table` delegates coercion ──────────────
     `actual` (vec_ptype2(new_table(1), new_table(FALSE))) not identical to `expected` (zap_dimnames(new_table(double()))).
    
     `dimnames(actual)` is a list
     `dimnames(expected)` is absent
     Backtrace:
     ▆
     1. └─vctrs:::expect_identical(...) at test-type-table.R:81:2
     2. └─vctrs:::expect_waldo_equal("identical", act, exp, info, ...) at tests/testthat/helper-vctrs.R:43:2
    
     [ FAIL 3 | WARN 0 | SKIP 308 | PASS 5289 ]
     Error: Test failures
     Execution halted
Flavor: [r-devel-linux-x86_64-fedora-clang](https://www.r-project.org/nosvn/R.check/r-devel-linux-x86_64-fedora-clang/vctrs-00check.html)

I have tracked this down to this undocumented R-devel change:

wch/r-source@2653cc6#diff-dbc5252913c71f32fa7295e497486c4c2b07ff43aa51b25c33e4dbdd397d3714R1249-R1253

// currently it is documented that `dim<-` removes dimnames() .. but ..
    SEXP odim = getAttrib0(vec, R_DimSymbol); // keep dimnames(.) if dim() entries are unchanged
    if((LENGTH(odim) != ndim) || memcmp((void *)INTEGER(odim),
					(void *)INTEGER(val), ndim * sizeof(int)))
	removeAttrib(vec, R_DimNamesSymbol);

This applies to dim<- and Rf_setAttrib(*, R_DimSymbol, *), which is what causes us issues.

---

Previously, in #1230 we were fairly aggressive about making the dimnames attribute returned from new_table() type stable, i.e. always a list() with either NULL or character vector elements.

This causes us some grief in vec_ptype2.table.table() because of the way Rf_setAttrib(*, R_DimSymbol, *) changed:

In R release:

• If vec_ptype2(tbl[, 2], tbl[, 2]) are combined, there is no dimnames attribute
• If vec_ptype2(tbl[, 1], tbl[, 2]) are combined, there is no dimnames attribute

In R-devel:

• If vec_ptype2(tbl[, 2], tbl[, 2]) are combined, we get list(NULL, NULL) dimnames (from our attempt to type stabilize them in new_table() + R-devel leaving them alone since the dimensions are the same)
• If vec_ptype2(tbl[, 1], tbl[, 2]) are combined, there is no dimnames attribute

I think the easiest thing to do is to just say that new_table() propagates NULL dimnames untouched, resulting in no dimnames attribute. I don't think vec_ptype2() should ever return any "names" related information, so this ensures we consistently don't add a dimnames attribute in R-devel and R-release.

[DavisVaughan]

R core ended up reverting this (thanks to Henrik on the mailing list) but I'm still going to merge because it is a candidate for further discussion and I think this PR improves things a little anyways wch/r-source@13f79d3