Fix custom-device default forwardPortSuccessRegex (#97719)

packages/flutter_tools/lib/src/commands/custom_devices.dart

@@ -736,11 +736,12 @@ class CustomDevicesAddCommand extends CustomDevicesCommandBase {
           '-o', 'ExitOnForwardFailure=yes',
           if (ipv6) '-6',
           '-L', '$formattedLoopbackIp:\${hostPort}:$formattedLoopbackIp:\${devicePort}',
-          sshTarget
+          sshTarget,
+          "echo 'Port forwarding success'; read"
         ]
         : null,
       forwardPortSuccessRegex: usePortForwarding
-        ? RegExp('Linux')
+        ? RegExp('Port forwarding success')
         : null,
 
       screenshotCommand: screenshotCommand.isNotEmpty

packages/flutter_tools/lib/src/custom_devices/custom_device_config.dart

@@ -264,8 +264,9 @@ class CustomDeviceConfig {
       '-o', 'ExitOnForwardFailure=yes',
       '-L', r'127.0.0.1:${hostPort}:127.0.0.1:${devicePort}',
       'pi@raspberrypi',
+      "echo 'Port forwarding success'; read",
     ],
-    forwardPortSuccessRegex: RegExp('Linux'),
+    forwardPortSuccessRegex: RegExp('Port forwarding success'),
     screenshotCommand: const <String>[
       'ssh',
       '-o', 'BatchMode=yes',
@@ -472,7 +473,7 @@ class CustomDeviceConfig {
       _kEnabled: enabled,
       _kPingCommand: pingCommand,
       _kPingSuccessRegex: pingSuccessRegex?.pattern,
-      _kPostBuildCommand: postBuildCommand,
+      _kPostBuildCommand: (postBuildCommand?.length ?? 0) > 0 ? postBuildCommand : null,
       _kInstallCommand: installCommand,
       _kUninstallCommand: uninstallCommand,
       _kRunDebugCommand: runDebugCommand,