return err if the spec area is nil after unmashal for tfjob v1alpha2

[jiaxuanzhou]

this is to fix: #641
func tfJobFromUnstructured may not recognize the spec area, tfjob.Spec.TFReplicaSpecs will be nil then, this logic should be considered to be err and stop sync.

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Coverage remained the same at 57.852% when pulling b6ec3e7 on jiaxuanzhou:fix into f405fb2 on kubeflow:master.

[gaocegege]

Please add some comments about the code.

For example, This is to close #641

Because the check may conflict ppl.

[gaocegege]

/ok-to-test

[gaocegege]

/lgtm
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[gaocegege]

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