fix(m2d): implement MSC4144 avatar clearing algorithm

- Empty string "" -> undefined (Discord uses default avatar)
- Valid MXC URI -> convert to public URL
- Omitted/null -> keep member avatar

src/m2d/converters/event-to-message.js

@@ -557,10 +557,18 @@ async function eventToMessage(event, guild, channel, di) {
 	const member = await getMemberFromCacheOrHomeserver(event.room_id, event.sender, di?.api)
 	if (member.displayname) displayName = member.displayname
 	if (member.avatar_url) avatarURL = mxUtils.getPublicUrlForMxc(member.avatar_url)
-	// Override display name and avatar from MSC4144 per-message profile if present
+	// MSC4144: Override display name and avatar from per-message profile if present
 	const perMessageProfile = event.content["com.beeper.per_message_profile"]
 	if (perMessageProfile?.displayname) displayName = perMessageProfile.displayname
-	if (perMessageProfile?.avatar_url) avatarURL = mxUtils.getPublicUrlForMxc(perMessageProfile.avatar_url)
+	if (perMessageProfile && "avatar_url" in perMessageProfile) {
+		if (perMessageProfile.avatar_url === "") {
+			// empty string avatar_url clears the avatar (use default)
+			avatarURL = undefined
+		} else if (perMessageProfile.avatar_url) {
+			// omitted/null falls back to member avatar
+			avatarURL = mxUtils.getPublicUrlForMxc(perMessageProfile.avatar_url)
+		}
+	}
 	// If the display name is too long to be put into the webhook (80 characters is the maximum),
 	// put the excess characters into displayNameRunoff, later to be put at the top of the message
 	let [displayNameShortened, displayNameRunoff] = splitDisplayName(displayName)

src/m2d/converters/event-to-message.test.js

@@ -5559,6 +5559,39 @@ test("event2message: com.beeper.per_message_profile overrides displayname and av
 	)
 })
 
+test("event2message: com.beeper.per_message_profile empty avatar_url clears avatar", async t => {
+	t.deepEqual(
+		await eventToMessage({
+			type: "m.room.message",
+			sender: "@cadence:cadence.moe",
+			content: {
+				msgtype: "m.text",
+				body: "hello with cleared avatar",
+				"com.beeper.per_message_profile": {
+					id: "no-avatar",
+					displayname: "No Avatar User",
+					avatar_url: ""
+				}
+			},
+			event_id: "$g07oYSZFWBkxohNEfywldwgcWj1hbhDzQ1sBAKvqOOU",
+			room_id: "!kLRqKKUQXcibIMtOpl:cadence.moe"
+		}),
+		{
+			ensureJoined: [],
+			messagesToDelete: [],
+			messagesToEdit: [],
+			messagesToSend: [{
+				username: "No Avatar User",
+				content: "hello with cleared avatar",
+				avatar_url: undefined,
+				allowed_mentions: {
+					parse: ["users", "roles"]
+				}
+			}]
+		}
+	)
+})
+
 test("event2message: data-mx-profile-fallback element is stripped from formatted_body when per-message profile is present", async t => {
 	t.deepEqual(
 		await eventToMessage({