fix workload reconcile error when revision enable trait is attached

[wonderflow]

the bug occurs when revisionEnabled trait is used.

Reproduce workflow, apply following yaml one by one:

apiVersion: core.oam.dev/v1alpha2
kind: TraitDefinition
metadata:
  annotations:
    definition.oam.dev/description: '`Scaler` is used to configure replicas for your
      service.'
  name: scaler
spec:
  definitionRef:
    name: manualscalertraits.core.oam.dev
  revisionEnabled: true
  workloadRefPath: spec.workloadRef

apiVersion: core.oam.dev/v1alpha2
kind: Component
metadata:
  name: example-component
spec:
  workload:
    apiVersion: core.oam.dev/v1alpha2
    kind: ContainerizedWorkload
    spec:
      containers:
        - name: wordpress
          image: wordpress:4.6.1-apache
          ports:
            - containerPort: 80
              name: wordpress
          env:
            - name: TEST_ENV
              value: test

apiVersion: core.oam.dev/v1alpha2
kind: ApplicationConfiguration
metadata:
  name: example-appconfig
spec:
  components:
    - componentName: example-component
      revisionName: example-component-v1
      traits:
        - trait:
            apiVersion: core.oam.dev/v1alpha2
            kind: ManualScalerTrait
            spec:
              replicaCount: 3

Then the appconfig will always reporting error:

  conditions:
  - lastTransitionTime: "2020-12-21T11:07:19Z"
    message: 'cannot apply components: cannot apply trait "core.oam.dev/v1alpha2"
      "ManualScalerTrait" "": cannot get object: resource name may not be empty'
    reason: ReconcileError
    status: "False"
    type: Synced
  dependency: {}
  observedGeneration: 1

The core reason is we fogort to add name when existing workload is exist:

This error happens but the workload is still created successfully, so our normal e2e test won't work. I wrote a new test for this case.

[codecov]

Codecov Report

Merging #797 (7609da2) into master (04868d2) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #797      +/-   ##
==========================================
+ Coverage   34.33%   34.52%   +0.19%     
==========================================
  Files          95       95              
  Lines        8399     8399              
==========================================
+ Hits         2884     2900      +16     
+ Misses       5191     5175      -16     
  Partials      324      324              

Flag	Coverage Δ
unittests	34.52% <100.00%> (+0.19%)	⬆️

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

Impacted Files	Coverage Δ
...am.dev/v1alpha2/applicationconfiguration/render.go	86.25% <100.00%> (+0.62%)	⬆️
...plicationconfiguration/applicationconfiguration.go	84.29% <0.00%> (+5.78%)	⬆️

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