functions/stdlib/merge: return an empty object when all inputs are null

[mildwonkey]

It seems intuitive for merge() to return an empty object when the inputs
are null (or omitted entirely) - returning an error would likely break
many terraform configurations, and returning an empty object rather than
null seems more consistent with the documented behavior (though it is
not explicit!).

I dug through the commit history and compared terraform's documentation,
post pre- and post-hcl2, and did not find any clues as to why it was
returning null rather than an empty object. I feel like this is an
acceptable breaking change, but am certainly open to other opinions.

[codecov]

Codecov Report

Merging #82 (a7d3a90) into main (4a53886) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
- Coverage   70.36%   70.31%   -0.06%     
==========================================
  Files          79       79              
  Lines        6550     6545       -5     
==========================================
- Hits         4609     4602       -7     
- Misses       1497     1498       +1     
- Partials      444      445       +1     

Impacted Files	Coverage Δ
cty/function/stdlib/collection.go	21.49% <ø> (-0.68%)	⬇️
cty/convert/mismatch_msg.go	69.38% <0.00%> (-2.05%)	⬇️

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[apparentlymart]

If I were reviewing this as a new function today I would've asked for it to return an error if any of the inputs are null, because that's typically how cty functions behave, but I agree that this seems like a reasonable compromise given the behavior as it currently exists, and so this is a pragmatic way to get a less surprising result.

Thanks!

[apparentlymart]

Hmm... this change was showing as having passed tests before I merged it but now I try it locally I'm seeing a test failure:

--- FAIL: TestMerge (0.00s)
    --- FAIL: TestMerge/merge([]cty.Value{cty.MapValEmpty(cty.String)}) (0.00s)
        collection_test.go:561: wrong result
            got:  cty.MapValEmpty(cty.String)
            want: cty.EmptyObjectVal

Unfortunately I spotted this only after including the change in the v1.7.1 release, so the "got" behavior is now contractual to avoid making a breaking change, so I think we must address this by changing the "want" to be an empty map too.

@mildwonkey would you mind sending in a new PR for that? The v1.7.1 release is out already but I'd like to make it green again in preparation for the next release. If you don't have time then please let me know and I can look into it myself.