CLEANUP: log: lf_text_len() returns a pointer not an integer

CLEANUP: log: lf_text_len() returns a pointer not an integer

In c83684519 ("MEDIUM: log: add the ability to include samples in logs")
we checked the return value of lf_text_len() as an integer instead of
comparing the pointer with NULL explicitly. Since this may be confusing,
let's test the return value against NULL.

[ada: for backports, the patch needs to be applied manually because of
 c6a713842 ("MINOR: log: simplify last_isspace in sess_build_logline()")]