Check for NULL prompt in readline_into_buffer()

Previously, passing readline() or readline_into_buffer() a NULL 'prompt'
parameter would result in puts() printing garbage when
CONFIG_CMDLINE_EDITING was enabled.

Note that no board currently triggers this bug.  Enabling
CONFIG_CMDLINE_EDITING on some boards (eg bab7xx) would result in
the bug appearing.  This change is only intended to prevent someone
from running into this issue in the future.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>