Disable send message when there's nothing to actually send

Now there's a way to check if the message content model has any useful
data inside of it.

BUG: 516118
FIXED-IN: 26.04

src/chatbar/SendBar.qml

@@ -208,6 +208,7 @@ RowLayout {
         icon.name: "document-send"
         text: i18nc("@action:button", "Send message")
         display: QQC2.AbstractButton.IconOnly
+        enabled: root.contentModel.hasAnyContent
 
         onClicked: root.contentModel.postMessage();
         QQC2.ToolTip.visible: hovered