Skip res_doh_send() if there is no validated DoH servers

Since res_doh_send() can take around 2 milliseconds, skip calling
it when there is no DoH servers validated.

Bug: 212364676
Test: ran "dnschk --attempts 1000 --randomname"
      w/o this change it takes 36 seconds
      w/ this change it takes 32 seconds
Test: checked the log to ensure that "performing query over Https"
      is not printed when the DNS doesn't support DoH
Change-Id: I1718534889b097eb89ba12a291da0fb19f13bbbd