test: Fail when an empty line is expected but not present

test: Fail when an empty line is expected but not present

The existing implementation of ut_assert_nextline_empty() cannot
distinguish between an empty line and no line at all. It can in fact be
called at the end of the recorded output and will happily return
success.

Adjust the logic so that this condition is detected. Show a failure
message in this case.

Fix the one test which falls foul of this fix.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 400175b0a7d ("test: Add a way to check each line of console...")
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>