fix Bookeeper npe (#107)

```
$ terraform import streamnative_pulsar_cluster.test-ursa max/fdl-ursa
streamnative_pulsar_cluster.test-ursa: Importing from ID "max/fdl-ursa"...
data.streamnative_pulsar_instance.test-ursa: Reading...
data.streamnative_pulsar_instance.test-ursa: Read complete after 1s [id=max/fdl-ursa]
╷
│ Error: Plugin did not respond
│ 
│ The plugin encountered an error, and failed to respond to the
│ plugin.(*GRPCProvider).ImportResourceState call. The plugin logs may contain more details.
╵


Stack trace from the terraform-provider-streamnative_v0.9.2 plugin:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x28 pc=0x1048162a0]

goroutine 181 [running]:
github.com/streamnative/terraform-provider-streamnative/cloud.convertCpuAndMemoryToStorageUnit(0x140003b2908?)
        github.com/streamnative/terraform-provider-streamnative/cloud/resource_pulsar_cluster.go:965 +0x20
github.com/streamnative/terraform-provider-streamnative/cloud.resourcePulsarClusterRead({0x105540168, 0x14000cd1200}, 0x1400028e680, {0x10537f9e0, 0x14000f16480})
        github.com/streamnative/terraform-provider-streamnative/cloud/resource_pulsar_cluster.go:635 +0x186c
github.com/streamnative/terraform-provider-streamnative/cloud.resourcePulsarCluster.func2({0x105540168?, 0x14000cd1200?}, 0x1400028e680, {0x10537f9e0?, 0x14000f16480?})
        github.com/streamnative/terraform-provider-streamnative/cloud/resource_pulsar_cluster.go:60 +0x30
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Provider).ImportState(0x14000864fc0, {0x105540168, 0x14000cd1200}, 0x14000d995d8, {0x14000f0d380, 0xc})
        github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/provider.go:388 +0x12c
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*GRPCProviderServer).ImportResourceState(0x14000c81248, {0x105540168?, 0x14000cd1140?}, 0x14000b39020)
        github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/grpc_provider.go:1097 +0xa4
github.com/hashicorp/terraform-plugin-go/tfprotov5/tf5server.(*server).ImportResourceState(0x140006d60a0, {0x105540168?, 0x14000cd0870?}, 0x1400017ad70)
        github.com/hashicorp/[email protected]/tfprotov5/tf5server/server.go:851 +0x188
github.com/hashicorp/terraform-plugin-go/tfprotov5/internal/tfplugin5._Provider_ImportResourceState_Handler({0x10539e260, 0x140006d60a0}, {0x105540168, 0x14000cd0870}, 0x1400028c9a0, 0x0)
        github.com/hashicorp/[email protected]/tfprotov5/internal/tfplugin5/tfplugin5_grpc.pb.go:440 +0x1c0
google.golang.org/grpc.(*Server).processUnaryRPC(0x14000b441e0, {0x10554eee0, 0x1400014e000}, 0x14000292a20, 0x140006247b0, 0x106d562b8, 0x0)
        google.golang.org/[email protected]/server.go:1335 +0xb90
google.golang.org/grpc.(*Server).handleStream(0x14000b441e0, {0x10554eee0, 0x1400014e000}, 0x14000292a20, 0x0)
        google.golang.org/[email protected]/server.go:1712 +0x7ac
google.golang.org/grpc.(*Server).serveStreams.func1.1()
        google.golang.org/[email protected]/server.go:947 +0xb4
created by google.golang.org/grpc.(*Server).serveStreams.func1 in goroutine 47
        google.golang.org/[email protected]/server.go:958 +0x14c

Error: The terraform-provider-streamnative_v0.9.2 plugin crashed!

This is always indicative of a bug within the plugin. It would be immensely
helpful if you could report the crash with the plugin's maintainers so that it
can be fixed. The output above should help diagnose the issue.
```

Author: labuladong

cloud/resource_pulsar_cluster.go

@@ -962,7 +962,7 @@ func convertCpuAndMemoryToComputeUnit(pc *cloudv1alpha1.PulsarCluster) float64 {
 }
 
 func convertCpuAndMemoryToStorageUnit(pc *cloudv1alpha1.PulsarCluster) float64 {
-	if pc != nil && pc.Spec.BookKeeper.Resources != nil {
+	if pc != nil && pc.Spec.BookKeeper != nil && pc.Spec.BookKeeper.Resources != nil {
 		cpu := pc.Spec.BookKeeper.Resources.Cpu.MilliValue()
 		memory := pc.Spec.BookKeeper.Resources.Memory.Value()
 		return math.Max(float64(cpu)/2/1000, float64(memory)/(8*1024*1024*1024))