Omit displayname or avatar_url if they aren't set instead of returning null (#7497)

Per https://github.com/matrix-org/matrix-doc/issues/1436#issuecomment-410089470 they should be omitted instead of returning null or "". They aren't marked as required in the spec.

Fixes https://github.com/matrix-org/synapse/issues/7333



Signed-off-by: Aaron Raimist <aaron@raim.ist>