Open onu adapter crashes during delete device

The open onu adapter will crash when the device is being deleted and the pm metrics manager is still not initialised. This happens when the device is deleted immediately after being discovered, where the activation is not yet done. This will cause the pm metrics manager to remain nil and when we delete, we try to cleanup pm metrics manager which will lead to crash.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1149cf3]

goroutine 1163091 [running]:
github.com/opencord/voltha-openonu-adapter-go/internal/pkg/pmmgr.(*OnuMetricsManager).SetdeviceDeletionInProgress(0x0, 0x1692601)
/go/src/internal/pkg/pmmgr/onu_metrics_manager.go:4013 +0x33
github.com/opencord/voltha-openonu-adapter-go/internal/pkg/core.(*OpenONUAC).DeleteDevice(0xc000802480, 0x1692680, 0xc083d3b260, 0xc084e0ac60, 0xc000802480, 0x1535978, 0xc0000da950)
/go/src/internal/pkg/core/openonu.go:338 +0x39f
github.com/opencord/voltha-protos/v5/go/adapter_service._AdapterService_DeleteDevice_Handler.func1(0x1692680, 0xc083d3b260, 0x14894e0, 0xc084e0ac60, 0x23fca60, 0x1692680, 0xc083d3b260, 0x16a3500)
/go/src/vendor/github.com/opencord/voltha-protos/v5/go/adapter_service/adapter_service.pb.go:747 +0x89
github.com/opencord/voltha-lib-go/v7/pkg/grpc.mkServerInterceptor.func1(0x1692680, 0xc083d3b260, 0x14894e0, 0xc084e0ac60, 0xc07f8649c0, 0xc04b0d8000, 0x0, 0x16a3500, 0x0, 0x0)
/go/src/vendor/github.com/opencord/voltha-lib-go/v7/pkg/grpc/server.go:158 +0x63
github.com/grpc-ecosystem/go-grpc-middleware.ChainUnaryServer.func1.1.1(0x1692680, 0xc083d3b260, 0x14894e0, 0xc084e0ac60, 0x14bb040, 0xd, 0x14fdab1, 0x2c)
/go/src/vendor/github.com/grpc-ecosystem/go-grpc-middleware/chain.go:25 +0x63
github.com/grpc-ecosystem/go-grpc-middleware/tracing/opentracing.UnaryServerInterceptor.func1(0x1692680, 0xc083d3b230, 0x14894e0, 0xc084e0ac60, 0xc07f8649c0, 0xc07f8649e0, 0xd2b4c6, 0x139c700, 0xc07f864a00, 0xc07f8649c0)