Apply factory method defaults to TableRepository/TablePartition constructors

[kzu]

When using the generic implementations via DI as follows:

 services.AddScoped(typeof(ITableRepository<>), typeof(TableRepository<>));
 services.AddScoped(typeof(ITablePartition<>), typeof(TablePartition<>));

The constructors would receive null values for things like the table name or partition key.
We should instead apply the same default value behavior we have in the corresponding
factory methods TableRepository.Create<T> and TablePartition<T> for consistency.