Fixed the white padding on top if desc is null

[AsjadSiddiqui]

For creating an alert with a gradient Using Container to fill the whole alert with 0 padding

Before this change:

After this change

Code to reproduce this alert:

final width = MediaQuery.of(context).size.width;
final height = MediaQuery.of(context).size.height;

Alert(
      context: context,
      type: AlertType.none,
      style: AlertStyle(
        buttonAreaPadding: EdgeInsets.zero,
        isCloseButton: false,
        isButtonVisible: false,
        alertBorder: RoundedRectangleBorder(
          borderRadius: BorderRadius.circular(0.0),
          side: BorderSide(
            color: Colors.grey,
          ),
        ),
        alertPadding:
            EdgeInsets.symmetric(horizontal: 20 / 375 * width, vertical: 0),
        constraints: BoxConstraints(
          minHeight: 0.6 * height,
          minWidth: 0.9 * width,
        ),
      ),
      padding: EdgeInsets.zero,
      content: Container(
        color: Colors.red,
        height: 0.6 * height,
        width: 0.95 * width,
        child: Column(
          children: <Widget>[],
        ),
      ),
      buttons: [],
    ).show();

[nguyendangkhoi]

please merge it

[ibrahimdevs]

@AsjadSiddiqui thanks for contribution!