Description of problem:
when configure github idp 'ca' field, the field not handled correctly in IDP config

Version-Release number of selected component (if applicable):
oc get clusterversion
NAME      VERSION                             
version   4.0.0-0.nightly-2019-01-29-025207 

How reproducible:
always

Steps to Reproduce:
1.when configure github idp 'ca' field is optional, but pod logs display `MountVolume.SetUp failed for volume "v4-0-config-user-idp-1--ca-crt" : configmap "v4-0-config-user-idp-1--ca-crt" not found`,so after edit oauth resource, the new authentication pod can not create successfully.And set 'ca: {}',still have this problem.

'''
apiVersion: config.openshift.io/v1
kind: OAuth
metadata:
  name: cluster
spec:
  identityProviders:
  - name: htpassidp
    challenge: true
    login: true
    mappingMethod: claim
    type: HTPasswd
    htpasswd:
      fileData:
        name: htpass-secret
  - name: github 
    challenge: false 
    login: true 
    mappingMethod: claim
    type: GitHub
    github:
      ca: {}
      clientID: c8ae7fa7fb268595719b 
      clientSecret: 
          name: my-secret 
'''
2.
3.

Actual results:
pod failed to create.

Expected results:
The pod created successfully.

Additional info:
Also tried the Google IDP, the clientSecret still could not mount to the openshift-authentication pod.