events: Allow the url in m.room.avatar to be null / missing

This isn't allowed in any version of the spec, but it's the only way to
unset an avatar and will have to be supported in the future.

C.f. https://github.com/matrix-org/matrix-doc/issues/2006

ruma-events/src/room/avatar.rs

@@ -14,19 +14,32 @@ pub type AvatarEvent = StateEvent<AvatarEventContent>;
 /// The payload for `AvatarEvent`.
 #[derive(Clone, Debug, Deserialize, Serialize, StateEventContent)]
 #[cfg_attr(not(feature = "unstable-exhaustive-types"), non_exhaustive)]
+#[cfg_attr(feature = "unstable-pre-spec", derive(Default))]
 #[ruma_event(type = "m.room.avatar")]
 pub struct AvatarEventContent {
     /// Information about the avatar image.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub info: Option<Box<ImageInfo>>,
 
     /// URL of the avatar image.
+    #[cfg(not(feature = "unstable-pre-spec"))]
     pub url: String,
+
+    /// URL of the avatar image.
+    #[cfg(feature = "unstable-pre-spec")]
+    pub url: Option<String>,
 }
 
 impl AvatarEventContent {
     /// Create an `AvatarEventContent` from the given image URL.
+    #[cfg(not(feature = "unstable-pre-spec"))]
     pub fn new(url: String) -> Self {
         Self { info: None, url }
     }
+
+    /// Create an empty `AvatarEventContent`.
+    #[cfg(feature = "unstable-pre-spec")]
+    pub fn new() -> Self {
+        Self::default()
+    }
 }