tests/functional: don't try to wait for the empty string

tests/functional: don't try to wait for the empty string

Telling exec_command_wand_wait_for_pattern to wait for the empty
string does not make any conceptual sense, as a check for empty
string will always succeed. It makes even less sense when followed
by a call to wait_for_console_pattern() with a real match.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20241121154218.1423005-14-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20241121165806.476008-14-alex.bennee@linaro.org>