set empty tags for conversations affected by #7178 (refs #7178)
set empty tags for conversations affected by #7178 (refs #7178)

recently created (or resurrected) conversations with no common
contexts (explicit or inferred) did not get the tags set to '',
and this was preventing the course filter from displaying in the
ui

test plan:

ensure that the course filter shows up

Change-Id: I2c332eedab7765a9d8e3bcb2f41cae260e89a611
Reviewed-on: https://gerrit.instructure.com/8542
Tested-by: Hudson <dfb64870173313a9a7b56f814c8e3b33e268497a@instructure.com>
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>

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