Fix Object GoString()

[mildwonkey]

The returned GoString() for a cty.ObjectWithOptionalAttrs was a bit off;
the returned slice of optional attributes was twice as long as it needed
to be due to a mismatch where the slice was created with make, but then
appended to (as opposed to indexed into).

I didn't see test coverage for Type GoString(), so I copied the test cases from Value's GoString() (fewer test cases, since I didn't need to differentiate between null and empty values). Here's the related test before the fix:

    --- FAIL: TestTypeGoString/cty.ObjectWithOptionalAttrs(map[string]cty.Type{"bar":cty.String,_"foo":cty.Bool},_[]string{"",_"bar"}) (0.00s)
        type_test.go:147: wrong result
            got:  cty.ObjectWithOptionalAttrs(map[string]cty.Type{"bar":cty.String, "foo":cty.Bool}, []string{"", "bar"})
            want: cty.ObjectWithOptionalAttrs(map[string]cty.Type{"bar":cty.String, "foo":cty.Bool}, []string{"bar"})

[codecov]

Codecov Report

Merging #86 (d5f83a1) into main (251ac0b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   70.64%   70.64%           
=======================================
  Files          79       79           
  Lines        6547     6547           
=======================================
  Hits         4625     4625           
  Misses       1478     1478           
  Partials      444      444           

Impacted Files	Coverage Δ
cty/object_type.go	78.26% <100.00%> (ø)

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[apparentlymart]

This looks great! Thanks for fixing this and sorry for the delay in reviewing.