fix creating certificates with a given csr referencing a ca issuer

[RaphaelVogel]

How to categorize this PR?
/kind bug

What this PR does / why we need it:
Creating a certificate referencing a ca issuer with a given csr creates a nil pointer exception.
Example:

apiVersion: cert.gardener.cloud/v1alpha1
kind: Certificate
metadata:
  name: cert-csr
  namespace: default
spec:
  csr: ......
  issuerRef:
    name: some-ca-issuer

Release note:

Creating certificates with a given csr referencing a ca issuer do not throw a nil pointer exception anymore 

[MartinWeindel]

/lgtm

We should introduce integration tests as a follow-up

[gardener-prow]

LGTM label has been added.

Git tree hash: 6786ea95723e51c15157639ae707d206c4874ef9

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: MartinWeindel

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [MartinWeindel]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[MartinWeindel]

/kind enhancement