Do not send empty certificates to nginx

[aledbf]

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

fixes #4106
replaces #4193

Special notes for your reviewer:

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1cd87b0). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master    #4229   +/-   ##
=========================================
  Coverage          ?   57.89%           
=========================================
  Files             ?       87           
  Lines             ?     6539           
  Branches          ?        0           
=========================================
  Hits              ?     3786           
  Misses            ?     2325           
  Partials          ?      428

Impacted Files	Coverage Δ
internal/ingress/sslcert.go	50% <ø> (ø)
internal/ingress/controller/nginx.go	28.72% <0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1cd87b0...225f881. Read the comment docs.

[ElvinEfendi]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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 [ElvinEfendi,aledbf]

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