Fix NULL placeholders in log entries

Fixed in this MR, please test/review.

While most of the places in the code concerned entity labels (search indexes in most cases, to be precise) which can be NULL theoretically, the most important changes were probably the ones concerning fields, in the AddHierarchy and RenderedItem processors, as field labels are probably more often NULL than entity labels.

Automatically closed - issue fixed for 2 weeks with no activity.