Fix reply colors being broken if you're faster than the server

This is that bug that causes reply colors to be white, and this error to
print in the log:

qrc:/qt/qml/org/kde/neochat/messagecontent/ReplyComponent.qml:41: TypeError: Cannot read property 'color' of null

The reason why this happens is inside of EventMessageContentModel, it
needs to be able to find the relevant event in the room to fetch the
room member (and then their color.) Dependent on many variables to
align, this can happen easily if you are faster than your server giving
you said events.

But this is an easy fix, we obviously get the event afterwards and just
need to re-evaluate the the author property. I also made sure it falls
back to some color instead of white, which will also quiet the error.

src/messagecontent/ReplyComponent.qml

@@ -38,7 +38,7 @@ RowLayout {
         Layout.fillHeight: true
 
         implicitWidth: Kirigami.Units.smallSpacing
-        color: root.replyContentModel.author.color
+        color: root.replyContentModel.author?.color ?? Kirigami.Theme.highlightColor
         radius: Kirigami.Units.cornerRadius
     }
     ColumnLayout {