handle null channel number

Emby.Server.Implementations/LiveTv/EmbyTV/EmbyTV.cs

@@ -847,12 +847,16 @@ namespace Emby.Server.Implementations.LiveTv.EmbyTV
 
                 var channelMappings = GetChannelMappings(provider.Item2);
                 var channelNumber = channel.Number;
+
+                if (!string.IsNullOrWhiteSpace(channelNumber))
+                {
                     string mappedChannelNumber;
                     if (channelMappings.TryGetValue(channelNumber, out mappedChannelNumber))
                     {
                         _logger.Debug("Found mapped channel on provider {0}. Tuner channel number: {1}, Mapped channel number: {2}", provider.Item1.Name, channelNumber, mappedChannelNumber);
                         channelNumber = mappedChannelNumber;
                     }
+                }
 
                 var programs = await provider.Item1.GetProgramsAsync(provider.Item2, channelNumber, channel.Name, startDateUtc, endDateUtc, cancellationToken)
                         .ConfigureAwait(false);