Check for null, strictly, in maniphest.update param validation

Summary:
If the first non-null entry in the params array is falsey, the request bombs.

Something like {"id":279,"projectPHIDs":[],"status":"0","ownerPHID":"PHID-USER-on3xxsnaljmfn36d4b7a"}

Test Plan:
Before:

  echo '{"id":279,"projectPHIDs":[],"status":"0","ownerPHID":"PHID-USER-cj3cpuh7oorbmnn2pl5g"}' | arc call-conduit maniphest.update
  {"error":"ERR-NO-EFFECT","errorMessage":"ERR-NO-EFFECT: Update has no effect.","response":null}

After:

  echo '{"id":279,"projectPHIDs":[],"status":"0","ownerPHID":"PHID-USER-cj3cpuh7oorbmnn2pl5g"}' | arc call-conduit maniphest.update
  {"error":null,"errorMessage":null,"response":{"id":"279","phid":"PHID-TASK-lbwcq3pmur2c5fuqqhlx"...

Reviewers: garoevans, epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8391