Hide row if data is empty in ListView

This is the appropriate case to use a ValueConverter.

All you need to do is create a converter like this:

namespace App.Converters
{
    public class TextToBoolConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if(value != null)
                if (!(value is string)) return true;
            return string.IsNullOrWhiteSpace(value as string) ? false : true;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

And use it on the IsVisible property of Label:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:converters="clr-namespace:App.Converters"
             x:Class="App.Views.SamplePage">
    <ContentPage.Resources>
        <ResourceDictionary>
            <converters:TextToBoolConverter x:Key="TextToBoolConverter" />
        </ResourceDictionary>
    </ContentPage.Resources>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="30*" />
            <ColumnDefinition Width="50*" />
        </Grid.ColumnDefinitions>
        <Label Text="{Binding EntryDate}" Grid.Column="0" Grid.Row="0"></Label>
        <Label Text="{Binding Sleep}" Grid.Column="1" Grid.Row="0"></Label>
        <Label Text="{Binding Comment}" Grid.Column="0" Grid.ColumnSpan="4" 
               Grid.Row="1"
               IsVisible={Binding Comment, Converter={StaticResource TextToBoolConverter}}/>
    </Grid>           
</ContentPage>

Thus you can reuse it wherever you need.