Avoid assert check when switching to main profile

This can happen if the user doesn't have an avatar set, as there's an
assert check inside of makeMediaUrl. We can avoid this by checking if
the string is empty before calling that.

src/app/qml/UserDetailDialog.qml

@@ -68,7 +68,14 @@ Kirigami.Dialog {
                 Layout.preferredHeight: Kirigami.Units.iconSizes.huge
 
                 name: root.room ? root.room.member(root.user.id).displayName : root.user.displayName
-                source: root.room ? root.room.member(root.user.id).avatarUrl : root.connection.makeMediaUrl(root.user.avatarUrl)
+                source: {
+                    if (root.room) {
+                        return root.room.member(root.user.id).avatarUrl;
+                    } else if(root.user.avatarUrl.toString() !== '') {
+                        return root.connection.makeMediaUrl(root.user.avatarUrl);
+                    }
+                    return "";
+                }
                 color: root.room ? root.room.member(root.user.id).color : QmlUtils.getUserColor(root.user.hueF)
             }