Marshaling and unmarshaling turns empty slice into nil

[bryantwr]

Hi there!

I'm finding that using marshal and unmarshal on an empty slice in a struct turns that value into a nil, which seems incorrect, given that I believe the Avro spec supports empty values. e.g.

func TestAvroEmpty(t *testing.T) {
	a := assert.New(t)
	type A struct {
		EmptySlice []string
	}
	mockA := A{
		EmptySlice: []string{},
	}
	marshalA, schemaA, err := avro.Marshal(mockA)
	a.Nil(err)

	var newA A
	_, err = avro.Unmarshal(marshalA, &newA, schemaA)
	a.Nil(err)

	diff := cmp.Diff(mockA, newA)
	a.Empty(diff)
}

yields the output:

=== RUN   TestA
...
        	Error:      	Should be empty, but was A{
        	            	- 	EmptySlice: []string{},
        	            	+ 	EmptySlice: nil,
        	            	  }
        	Test:       	TestA
--- FAIL: TestA (0.00s)

Is this expected behavior for the library?

[skateinmars]

Hey @bryantwr ,
this does look like an issue, especially since in that case the array is not optional

From what I understand from the Avro specs nil arrays should only happen when the field is a union of ["null", array] and the value is actually null

We have a limited bandwidth to work on this but feel free to dig into the code and open a PR!