Author custom JSON marshaller functions to "omitempty" structs

[mrutkows]

See issue #13

As CycloneDX schema has many nested structs which are not, by default, removed if the are "empty" on output (--format json), we would need to implement (on a pre-struct basis) a custom marshaller (i.e., json.Marshaller) which allows us to decide which members of the struct should be omitted.

This is important for the LicenseChoice (specifically we need to remove the CDXAttachment child struct) and Vulnerability (specifically CDXAffect) structs to reduce output sizes.

Note: The CDXAttachment struct is reused elsewhere so the benefits should be seen in many places.

[mrutkows]

See: https://stackoverflow.com/questions/46265751/omitting-json-for-empty-custom-type

if you really want to omit a field based and internal state, you need to implement json.Marshaller on your structure, not its children. The easiest way to do this would be as follows:

func (d Data) MarshalJSON() ([]byte, error) {
    if !d.Stderr.Valid {
        return json.Marshal(Data{d.X, d.Y, nil})
    }
    return json.Marshal(d)
}

[mrutkows]

more help... https://stackoverflow.com/questions/65200070/how-to-marshal-nested-struct-to-flat-json

type A struct {
    Value float64
    Unit  string
}

type B struct {
    a1 A
    a2 A
}

func (b *B) MarshalJSON() ([]byte, error) {
    intermediate := map[string]interface{}{
        "a1":   b.a1.Value,
        "a1_u": b.a1.Unit,
        "a2":   b.a2.Value,
        "a2_u": b.a2.Unit,
    }
    return json.Marshal(intermediate)
}

[mrutkows]

Fixed by #22