sink.text.details: don't write empty strings

sink.text.details: don't write empty strings

This patch makes a `sink.text.details` not write the value of a trace
environment entry or of a value object when it's an empty string.

This is mostly because I don't like to keep plain text files with
significant trailing whitespaces in this repository, and
`data/plugins/src.ctf.lttng-live/cli-multi-domains.expect` had some,
for example:

    trace_creation_datetime:️▒
    trace_name:▒

where `▒` is a space.

Of course we'll have the same issue if a user string contains only
whitespaces instead of being empty, but since it's unlikely that we'll
do this in our own tests, this patch is good enough for me.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I6be83e2721bc6ce98e1459a7df42e32ccf6b661d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12249
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>