bugfix: fields with tag "write_empty" are not marshaled correctly

[rickyyangz]

as the titled described, the implementation of write_empty is not correct, fields with write_empty tag may still be marshaled to empty bytes.

func TestForceWriteEmpty(t *testing.T) {
type InnerWriteEmpty struct {
    // sth. that isn't zero-len if default, e.g. fixed32:
    ValIn int32 `amino:"write_empty" binary:"fixed32"`
}

type OuterWriteEmpty struct {
    In  InnerWriteEmpty `amino:"write_empty"`
    Val int             `amino:"write_empty" binary:"fixed32"`
}

cdc := amino.NewCodec()

b, err := cdc.MarshalBinaryBare(OuterWriteEmpty{})
assert.NoError(t, err)
assert.Equal(t, []byte{10, 5, 13, 0, 0, 0, 0, 16, 0}, b)

b, err = cdc.MarshalBinaryBare(InnerWriteEmpty{})
assert.NoError(t, err)
assert.Equal(t, []byte{13, 0, 0, 0, 0,}, b)
}

the result is

Error:          Not equal: 
                        expected: []byte{0xa, 0x5, 0xd, 0x0, 0x0, 0x0, 0x0, 0x10, 0x0}
                        actual  : []byte{0xa, 0x5, 0xd, 0x0, 0x0, 0x0, 0x0}

Error:          Not equal: 
                        expected: []byte{0xd, 0x0, 0x0, 0x0, 0x0}
                        actual  : []byte(nil)

[rickyyangz]

btw, I submitted a report to the bug bounty program

[zmanian]

We should consider back porting this fix to v0.15 and consider doing v0.16 release to fix this.

@rickyyangz is this bug blocking you?