BUG/MINOR: log: fix lf_text() behavior with empty string

BUG/MINOR: log: fix lf_text() behavior with empty string

As reported by Baptiste in GH #2797, if a logformat alias leveraging
lf_text() ends up printing nothing (empty string), the whole logformat
evaluation stops, leading garbage log message.

This bug was introduced during 3.0 cycle in fcb7e4b ("MINOR: log: add
lf_rawtext{_len}() functions"). At that time I genuinely thought that
if strlcpy2() returned 0, it was due to a lack of space, actually
forgetting that the function may simply be called with an empty string.

Because of that, lf_text() would return NULL if called with an empty
string, and since all lf_*() helpers are expected to return NULL on
error, this explains why the logformat evaluation immediately stops in
this case.

To fix the issue, let's simply consider that strlcpy2() returning 0 is
not an error, like it was already the case before.

It should be backported in 3.1 and 3.0 with fcb7e4b.