🐛 Handle GetCluster == nil in CRD uws

[m-messiah]

What this PR does / why we need it:

c.MultiClusterController.GetCluster method returns a pointer, but nil check was not done, so sometimes we could see the panic in logs, resulting syncer to be restarted or completely broken in that loop.

E0915 11:20:03.699391       1 runtime.go:78] Observed a panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)
goroutine 6354 [running]:
k8s.io/apimachinery/pkg/util/runtime.logPanic({0x1b6d040?, 0x30ce4b0})
    /go/pkg/mod/k8s.io/apimachinery@v0.21.1/pkg/util/runtime/runtime.go:74 +0x99
k8s.io/apimachinery/pkg/util/runtime.HandleCrash({0x0, 0x0, 0xc18faa20d0?})
    /go/pkg/mod/k8s.io/apimachinery@v0.21.1/pkg/util/runtime/runtime.go:48 +0x75
panic({0x1b6d040, 0x30ce4b0})
    /usr/local/go/src/runtime/panic.go:884 +0x212
sigs.k8s.io/cluster-api-provider-nested/virtualcluster/pkg/syncer/resources/crd.(*controller).BackPopulate(0xc0007f4480, {0xc18836d5e0?, 0xc18faa20d0?})
    /go/pkg/mod/sigs.k8s.io/cluster-api-provider-nested/virtualcluster@v0.0.0-20220825091205-ef058637a56e/pkg/syncer/resources/crd/uws.go:67 +0x149
...

The PR just adds a non-nil check for the object as it is already done in other invocations of the method

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: christopherhein, m-messiah

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