Fix unnecessary warning when PV bound to PVC is not created yet

[takuhiro]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR adds the empty check of pvc.Spec.VolumeName before getting the PV to avoid an unnecessary warning here that is logged every time a new PVC is created.

before:

I0816 07:50:11.380497       1 controller.go:291] Started PVC processing "default/sample-volume-jtdql"
W0816 07:50:11.380594       1 controller.go:318] PV "" bound to PVC default/sample-volume-jtdql not found

after:

I0818 08:00:08.852855       1 controller.go:291] Started PVC processing "default/sample-volume-v2784"
I0818 08:00:08.855161       1 controller.go:314] PV bound to PVC "default/sample-volume-v2784" is not created yet

Which issue(s) this PR fixes:

Fixes #170

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Fix unnecessary warning when PV bound to PVC is not created yet.

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[k8s-ci-robot]

Hi @takuhiro. Thanks for your PR.

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[gnufied]

/ok-to-test

[takuhiro]

@gnufied Thank you for running tests. Could you merge this?

[takuhiro]

@gnufied @jingxu97 @humblec Could you approve and merge this PR?

[gnufied]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gnufied, takuhiro

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