Publishing fails at Publishing Mappings TBB because of missing null/empty string check in codebase

[dmandava]

https://github.com/sdl/dxa-content-management/blob/1959468d0175d6b9ad843e8969346c17b58f5583/Sdl.Web.Tridion.Templates/Templates/PublishMappings.cs#L152

DXA 1.6

Problem
ExtractTypeOfAppData method returns empty string if the schema does not have AppData for ApplicationId 'TypeOf'. With existing code the variable typeOf will be "tri:Content,".
In BuildSchemaSemanticsJson method with "input = tri:Content," results in an error.

This results in publishing failure of Publish Settings page.

Existing code
PublishingMappings.cs Line 150
ApplicationData appData = schema.LoadApplicationData(TypeOfAppDataId);
if (appData != null)
{
typeOf += "," + ExtractTypeOfAppData(appData);
}

Solution
We have to check to see if ExtractTypeOfAppData method is not empty, then only add ',' separated values of typeOf values.

            if (appData != null)
            {
                string typeOfAppData = ExtractTypeOfAppData(appData);
                if (!string.IsNullOrEmpty(typeOfAppData)) //new line to be added to avoid 
                {
                    typeOf += "," + ExtractTypeOfAppData(appData);
                }                    
            }