if sourceworkload is nil, not print metrics

[LiZhenCheng9527]

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes:
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[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 54.45%. Comparing base (739fde5) to head (1f2940b).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/controller/telemetry/metric.go	25.00%	5 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
pkg/controller/telemetry/metric.go	55.66% <25.00%> (-0.71%)	⬇️

... and 6 files with indirect coverage changes

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[hzxuzhonghu]

/lgtm

[kmesh-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hzxuzhonghu

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