cli: Avoid storing empty lines in command history

When the console manager is run in the interactive mode, it also saves
empty lines (i.e. the Enter key is pressed without running a command) in
it's command history. Avoid this by processing the line only if
readline() returns a non-empty string. Makes it easier to navigate the
history using arrow keys.

	modified:   cli/src/cli-rl.c

Change-Id: I0fcce394474589bb345b7c9ef39d25849dc0c2af
BUG: 957139
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/4894
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>