fix(container_node_pool): panic interface conversion on linux_node_config.sysctls

[modular-magician]

Fixes: hashicorp/terraform-provider-google#15847

• Add type assertion to extract empty values from linux_node_config.sysctls field
• Clean up existing test case to only pass single sysctls parameter to be changed
• Add test case to create a nodepool with empty linux_node_config.sysctls

Without the fix, the test case to create a nodepool with empty linux_node_config.sysctls will fail.

❯ make testacc TEST=./google/services/container TESTARGS='-run=TestAccContainerNodePool_withLinuxNodeConfig'
TF_ACC=1 TF_SCHEMA_PANIC_ON_ERROR=1 go test ./google/services/container -v -run=TestAccContainerNodePool_withLinuxNodeConfig -timeout 240m -ldflags="-X=github.com/hashicorp/terraform-provider-google/version.ProviderVersion=acc"
=== RUN   TestAccContainerNodePool_withLinuxNodeConfig
=== PAUSE TestAccContainerNodePool_withLinuxNodeConfig
=== CONT  TestAccContainerNodePool_withLinuxNodeConfig
panic: interface conversion: interface {} is nil, not map[string]interface {}

goroutine 1276 [running]:
github.com/hashicorp/terraform-provider-google/google/services/container.expandLinuxNodeConfig(...)
	/Users/tsubasanagasawa/go/src/github.com/hashicorp/terraform-provider-google/google/services/container/node_config.go:927
github.com/hashicorp/terraform-provider-google/google/services/container.expandNodeConfig({0x1071ec840?, 0x14001a923d8?})
	/Users/tsubasanagasawa/go/src/github.com/hashicorp/terraform-provider-google/google/services/container/node_config.go:849 +0x1808
github.com/hashicorp/terraform-provider-google/google/services/container.expandNodePool(0x1079eb778?, {0x0, 0x0})
	/Users/tsubasanagasawa/go/src/github.com/hashicorp/terraform-provider-google/google/services/container/resource_container_node_pool.go:867 +0x2f8
github.com/hashicorp/terraform-provider-google/google/services/container.resourceContainerNodePoolCreate(0x140029c3380, {0x107989740?, 0x14000f32400?})
	/Users/tsubasanagasawa/go/src/github.com/hashicorp/terraform-provider-google/google/services/container/resource_container_node_pool.go:477 +0x120
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).create(0x1079e2678?, {0x1079e2678?, 0x1400140e630?}, 0xd?, {0x107989740?, 0x14000f32400?})
	/Users/tsubasanagasawa/go/pkg/mod/github.com/hashicorp/terraform-plugin-sdk/v2@v2.24.0/helper/schema/resource.go:695 +0x134
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).Apply(0x140007ad500, {0x1079e2678, 0x1400140e630}, 0x140019cbc70, 0x140029c3200, {0x107989740, 0x14000f32400})
	/Users/tsubasanagasawa/go/pkg/mod/github.com/hashicorp/terraform-plugin-sdk/v2@v2.24.0/helper/schema/resource.go:837 +0x888
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*GRPCProviderServer).ApplyResourceChange(0x14000eab110, {0x1079e2678?, 0x1400140e570?}, 0x140024650e0)
	/Users/tsubasanagasawa/go/pkg/mod/github.com/hashicorp/terraform-plugin-sdk/v2@v2.24.0/helper/schema/grpc_provider.go:1021 +0xb78
github.com/hashicorp/terraform-plugin-mux/tf5muxserver.muxServer.ApplyResourceChange({0x140010720c0, 0x14001072120, {0x1400230eee0, 0x2, 0x2}, {0x0, 0x0, 0x0}, {0x0, 0x0, ...}, ...}, ...)
	/Users/tsubasanagasawa/go/pkg/mod/github.com/hashicorp/terraform-plugin-mux@v0.8.0/tf5muxserver/mux_server_ApplyResourceChange.go:27 +0xdc
github.com/hashicorp/terraform-plugin-go/tfprotov5/tf5server.(*server).ApplyResourceChange(0x140024483c0, {0x1079e2678?, 0x140013d1d40?}, 0x1400196c770)
	/Users/tsubasanagasawa/go/pkg/mod/github.com/hashicorp/terraform-plugin-go@v0.14.3/tfprotov5/tf5server/server.go:818 +0x3bc
github.com/hashicorp/terraform-plugin-go/tfprotov5/internal/tfplugin5._Provider_ApplyResourceChange_Handler({0x107904520?, 0x140024483c0}, {0x1079e2678, 0x140013d1d40}, 0x1400196c700, 0x0)
	/Users/tsubasanagasawa/go/pkg/mod/github.com/hashicorp/terraform-plugin-go@v0.14.3/tfprotov5/internal/tfplugin5/tfplugin5_grpc.pb.go:385 +0x170
google.golang.org/grpc.(*Server).processUnaryRPC(0x14000c22000, {0x1079eb898, 0x140027c7380}, 0x140019bdb00, 0x14001b552c0, 0x108f5a8e0, 0x0)
	/Users/tsubasanagasawa/go/pkg/mod/google.golang.org/grpc@v1.57.0/server.go:1360 +0xcb4
google.golang.org/grpc.(*Server).handleStream(0x14000c22000, {0x1079eb898, 0x140027c7380}, 0x140019bdb00, 0x0)
	/Users/tsubasanagasawa/go/pkg/mod/google.golang.org/grpc@v1.57.0/server.go:1737 +0x82c
google.golang.org/grpc.(*Server).serveStreams.func1.1()
	/Users/tsubasanagasawa/go/pkg/mod/google.golang.org/grpc@v1.57.0/server.go:982 +0x84
created by google.golang.org/grpc.(*Server).serveStreams.func1
	/Users/tsubasanagasawa/go/pkg/mod/google.golang.org/grpc@v1.57.0/server.go:980 +0x16c
FAIL	github.com/hashicorp/terraform-provider-google/google/services/container	331.318s
FAIL

Release Note Template for Downstream PRs (will be copied)

container: fixed an issue in `google_container_node_pool` where empty `linux_node_config.sysctls` would crash the provider

Derived from GoogleCloudPlatform/magic-modules#8981