Fix swipe-back triggering unwanted draft deletion signing

When swiping back on a DM screen with an empty message, the BackHandler
was calling sendDraftSync() which checks if the message is blank and then
calls deleteDraftIgnoreErrors(). This creates and signs a NIP-09 DeletionEvent,
prompting the external signer to sign a deletion event.

The fix adds a check to only call sendDraftSync() when there's actual
content in the message, avoiding the unnecessary signing prompt.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/privateDM/send/PrivateMessageEditFieldRow.kt

@@ -88,10 +88,12 @@ fun PrivateMessageEditFieldRow(
     nav: INav,
 ) {
     BackHandler {
-        accountViewModel.launchSigner {
-            channelScreenModel.sendDraftSync()
-            channelScreenModel.cancel()
+        if (channelScreenModel.message.text.isNotBlank()) {
+            accountViewModel.launchSigner {
+                channelScreenModel.sendDraftSync()
+            }
         }
+        channelScreenModel.cancel()
         nav.popBack()
     }