display "(no subject)" for conversations w/o subjects.
display "(no subject)" for conversations w/o subjects.

fixes CNVS-7433

test plan:
  * create a conversation without a subject;
  * navigate to the new conversations interface;
  * verify that both on the conversation detail and in the
    conversation list on the left of the interface display
    "(No subject)".

Change-Id: Id1ab9077ff19c690f3449a591c68860411a90e65
Reviewed-on: https://gerrit.instructure.com/23219
QA-Review: Cam Theriault <b4133c36b6d893d0872cc09e87b536d16aa2d241@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Braden Anderson <a6cc2cef7d25f27847876bdb81651755fcebacf0@instructure.com>
Product-Review: Zach Pendleton <514dbb4d54d4f29e454990df7d075678f43f92aa@instructure.com>

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