BUG/MINOR: stats: make field_str() return an empty string on NULL

BUG/MINOR: stats: make field_str() return an empty string on NULL

It already returns an empty string when the field is empty, but as a
preventive measure we should do the same when the string itself is a
NULL. While it is not supposed to happen, it will make the code more
resistant against failed allocations and unexpected results.

This fix should be backported to 1.7.