fix panic when printing log when lastSuccessfulTime of cronjob is nil

[chaunceyjiang]

What type of PR is this?
/kind bug

What this PR does / why we need it:

fix aggregateCronJobStatus panic

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

When job is not successful even once, lastSuccessfulTime will always benil. Printing logs at this time will cause a panic.

Does this PR introduce a user-facing change?:

`karmada-controller-manager`: Fixed panic when printing log in case lastSuccessfulTime of cronjob is nil.

[chaunceyjiang]

/cc @XiShanYongYe-Chang

[codecov-commenter]

Codecov Report

Merging #3683 (c3a4c26) into master (e95c605) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@            Coverage Diff             @@
##           master    #3683      +/-   ##
==========================================
- Coverage   56.63%   56.61%   -0.02%     
==========================================
  Files         221      221              
  Lines       20831    20831              
==========================================
- Hits        11797    11793       -4     
- Misses       8411     8414       +3     
- Partials      623      624       +1     

Flag	Coverage Δ
unittests	56.61% <100.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ourceinterpreter/default/native/aggregatestatus.go	70.80% <100.00%> (-0.69%)	⬇️

... and 1 file with indirect coverage changes

[XiShanYongYe-Chang]

Oh, thanks~
How do you find it?
/lgtm
/approve

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: XiShanYongYe-Chang

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:

• pkg/resourceinterpreter/OWNERS [XiShanYongYe-Chang]

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

[XiShanYongYe-Chang]

Can you help cherry-pick it to the affected release branch?

[chaunceyjiang]

How do you find it?

Through the logging system.