[BUG] Update status.mountTime failed because of null value in K8s 1.18

[TrafalgarZZZ]

What is your environment(Kubernetes version, Fluid version, etc.)
Fluid release 0.7 3d66068

Describe the bug
When following the instructions in juicefs runtime demo, it fails with the following error in juicefsruntime-controller:

2022-03-19T17:21:11.519+0800    ERROR   juicefsctl.JuiceFSRuntime       base/setup.go:39        Update runtime status   {"juicefsruntime": "default/jfsdemo", "error": "JuiceFSRuntime.data.fluid.io \"jfsdemo\" is invalid: status.mountTime: Invalid value: \"null\": status.mountTime in body must be of type string: \"null\""}
github.com/fluid-cloudnative/fluid/vendor/github.com/go-logr/zapr.(*zapLogger).Error
        /go/src/github.com/fluid-cloudnative/fluid/vendor/github.com/go-logr/zapr/zapr.go:132
github.com/fluid-cloudnative/fluid/pkg/ddc/juicefs.JuiceFSEngine.SetupMaster
        /go/src/github.com/fluid-cloudnative/fluid/pkg/ddc/juicefs/master.go:104
github.com/fluid-cloudnative/fluid/pkg/ddc/base.(*TemplateEngine).Setup
        /go/src/github.com/fluid-cloudnative/fluid/pkg/ddc/base/setup.go:39
github.com/fluid-cloudnative/fluid/pkg/controllers.(*RuntimeReconciler).ReconcileRuntime
        /go/src/github.com/fluid-cloudnative/fluid/pkg/controllers/runtime_controller.go:219
github.com/fluid-cloudnative/fluid/pkg/controllers.(*RuntimeReconciler).ReconcileInternal
        /go/src/github.com/fluid-cloudnative/fluid/pkg/controllers/runtime_controller.go:139
github.com/fluid-cloudnative/fluid/pkg/controllers/v1alpha1/juicefs.(*JuiceFSRuntimeReconciler).Reconcile
        /go/src/github.com/fluid-cloudnative/fluid/pkg/controllers/v1alpha1/juicefs/juicefsruntime_controller.go:96

Actually, it's a common issue for all the runtime, but now it only affects JuiceFsRuntime. I think the reason is that only juiceFSRuntime CRD is up-to-date and contains the status.mountTime property.

You can find the property in v0.7.0 juicefsruntime crd:
https://github.com/fluid-cloudnative/fluid/blob/master/charts/fluid/v0.7.0/crds/data.fluid.io_juicefsruntimes.yaml#L1085

While you can't find it in other runtime crd:
https://github.com/fluid-cloudnative/fluid/blob/master/charts/fluid/v0.7.0/crds/data.fluid.io_alluxioruntimes.yaml

What you expect to happen:

How to reproduce it
juicefs runtime demo

Additional Information
Related PR:
#1340

[TrafalgarZZZ]

This problem is fixed in K8s 1.20:

kubernetes/kubernetes#86811 (comment)

While to fully support lower version K8s like 1.18, a possible solution may be changing metav1.Time to *metav1.Time

kubernetes/kubernetes#86811 (comment)