TidbCluster cannot be deployed when config is nil

Bug Report

What version of Kubernetes are you using?

client: v1.19.4
server: v1.18.2

What version of TiDB Operator are you using?

master branch

What storage classes exist in the Kubernetes cluster and what are used for PD/TiKV pods?

not related

What's the status of the TiDB cluster pods?

pd pod status is Pending

What did you do?

apply tc with definition:

apiVersion: pingcap.com/v1alpha1
kind: TidbCluster
metadata:
  name: basic
spec:
  version: v4.0.10
  timezone: UTC
  pvReclaimPolicy: Delete
  configUpdateStrategy: RollingUpdate
  discovery: {}
  pd:
    baseImage: pingcap/pd
    replicas: 1
    requests:
      storage: "1Gi"
  tikv:
    baseImage: pingcap/tikv
    replicas: 1
    requests:
      storage: "1Gi"
    config: {}
  tidb:
    baseImage: pingcap/tidb
    replicas: 1
    config: {}

What did you expect to see?
pd pod event

Events:
  Type     Reason       Age                 From               Message
  ----     ------       ----                ----               -------
  Normal   Scheduled    106s                default-scheduler  Successfully assigned default/basic-pd-0 to kind-control-plane
  Warning  FailedMount  43s (x8 over 106s)  kubelet            MountVolume.SetUp failed for volume "startup-script" : configmap "basic-pd" not found
  Warning  FailedMount  43s (x8 over 106s)  kubelet            MountVolume.SetUp failed for volume "config" : configmap "basic-pd" not found

What did you see instead?
tidb cluster deployed.

Reproduce the issue with the TidbCluster YAML in the report using the stated Kubernetes and TiDB Operator versions, then inspect the PD pod events for missing basic-pd ConfigMaps. Trace the existing TidbCluster deployment path for nil versus empty config; done means a cluster with nil config deploys without FailedMount events.