return None rather than raising exception if displayname/avatar_url empty
return None rather than raising exception if displayname/avatar_url empty

It's legal for them to be empty:

  "The user's display name (avatar URL) if they have set one, otherwise not
  present."

See
https://matrix.org/docs/spec/client_server/r0.2.0.html#get-matrix-client-r0-profile-userid-displayname
https://matrix.org/docs/spec/client_server/r0.2.0.html#get-matrix-client-r0-profile-userid-avatar-url

dovf/matrix-python-sdk,matrix-org/matrix-python-sdk