BUG/MINOR: debug: do not emit empty lines in thread dumps

BUG/MINOR: debug: do not emit empty lines in thread dumps

In 2.3, commit 471425f51 ("BUG/MINOR: debug: Don't dump the lua stack
if it is not initialized") introduced the possibility to emit an empty
line when there's no Lua info to dump. The problem is that doing this
on the CLI in "show threads" marks the end of the output, and it may
affect some external tools. We need to make sure that LFs are only
emitted if there's something on the line and that all lines properly
start with the prefix.

This may be backported as far as 2.0 since the commit above was
backported there.