flow: Make `value` property of MultilineInput not required
flow: Make `value` property of MultilineInput not required

The component does not require it to function porperly.

We are also moving its only usage (in the compose box) to an
non-controlled component.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile