Do not try to add empty attachments to eml export (#4880)

The upstream library we are currently using errors out if we try to attach an empty file. This PR skips trying to attach empty files.

<!-- PR description-->

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* #<issue>

#### Test Plan

<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/converters/eml/eml.go

@@ -157,6 +157,16 @@ func FromJSON(ctx context.Context, body []byte) (string, error) {
 				return "", clues.WrapWC(ctx, err, "invalid content bytes")
 			}
 
+			if len(bts) == 0 {
+				// TODO(meain): pass the data through after
+				// https://github.com/xhit/go-simple-mail/issues/96
+				logger.Ctx(ctx).
+					With("attachment_id", ptr.Val(attachment.GetId())).
+					Info("empty attachment")
+
+				continue
+			}
+
 			name := ptr.Val(attachment.GetName())
 
 			contentID, err := attachment.GetBackingStore().Get("contentId")

src/internal/converters/eml/eml_test.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/jhillyerd/enmime"
+	kjson "github.com/microsoft/kiota-serialization-json-go"
 	"github.com/microsoftgraph/msgraph-sdk-go/models"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -129,3 +130,32 @@ func (suite *EMLUnitSuite) TestConvert_messageble_to_eml() {
 
 	assert.Equal(t, source, target)
 }
+
+func (suite *EMLUnitSuite) TestConvert_empty_attachment_no_err() {
+	t := suite.T()
+
+	ctx, flush := tester.NewContext(t)
+	defer flush()
+
+	body := []byte(testdata.EmailWithAttachments)
+
+	msg, err := api.BytesToMessageable(body)
+	require.NoError(t, err, "creating message")
+
+	attachments := msg.GetAttachments()
+	err = attachments[0].GetBackingStore().Set("contentBytes", []uint8{})
+	require.NoError(t, err, "setting content bytes")
+
+	writer := kjson.NewJsonSerializationWriter()
+
+	defer writer.Close()
+
+	err = writer.WriteObjectValue("", msg)
+	require.NoError(t, err, "serializing message")
+
+	nbody, err := writer.GetSerializedContent()
+	require.NoError(t, err, "getting serialized content")
+
+	_, err = FromJSON(ctx, nbody)
+	assert.NoError(t, err, "converting to eml")
+}