consul_config_entry_service_intentions: NPE when optional JWT unset, causes panic

[Esity]

Per sundbry jwt is an optional parameter on the service intention config. When it was not set, the plugin would segfault due to a null pointer dereference on the ServiceIntentionsConfigEntry.JWT.

This adds a check for si.JWT != nil and skips building the jwt payload when it is not set.

Stack trace observed:

Stack trace from the terraform-provider-consul plugin:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xf0eb82]

goroutine 49 [running]:
github.com/hashicorp/terraform-provider-consul/consul.(*serviceIntentions).Write(0xc000b97598?, {0x14eb950?, 0xc000813100?}, 0xc00078c150?, 0x12?)
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/consul/resource_consul_config_entry_service_intentions.go:441 +0x3a2
github.com/hashicorp/terraform-provider-consul/consul.configEntryImplementationRead.func1(0xc0007a0fc0, {0x102a6a0?, 0xc00041c2a0?})
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/consul/resource_consul_config_entry_concrete.go:111 +0x202
github.com/hashicorp/terraform-provider-consul/consul.configEntryImplementationWrite.func1(0x10442e0?, {0x102a6a0, 0xc00041c2a0})
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/consul/resource_consul_config_entry_concrete.go:85 +0x2fd
github.com/hashicorp/terraform-plugin-sdk/helper/schema.(*Resource).Apply(0xc0003de500, 0xc000781720, 0xc00010f680, {0x102a6a0, 0xc00041c2a0})
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/vendor/github.com/hashicorp/terraform-plugin-sdk/helper/schema/resource.go:320 +0x438
github.com/hashicorp/terraform-plugin-sdk/helper/schema.(*Provider).Apply(0xc00038a680, 0xc0005bf8f0, 0x11c25c3?, 0xf?)
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/vendor/github.com/hashicorp/terraform-plugin-sdk/helper/schema/provider.go:294 +0x70
github.com/hashicorp/terraform-plugin-sdk/internal/helper/plugin.(*GRPCProviderServer).ApplyResourceChange(0xc000015240, {0xc0007a0a80?, 0x4c4e86?}, 0xc0007a0a80)
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/vendor/github.com/hashicorp/terraform-plugin-sdk/internal/helper/plugin/grpc_provider.go:895 +0x805
github.com/hashicorp/terraform-plugin-sdk/internal/tfplugin5._Provider_ApplyResourceChange_Handler({0x1177e00?, 0xc000015240}, {0x14e77d0, 0xc000793f80}, 0xc0007a0a10, 0x0)
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/vendor/github.com/hashicorp/terraform-plugin-sdk/internal/tfplugin5/tfplugin5.pb.go:3305 +0x170
google.golang.org/grpc.(*Server).processUnaryRPC(0xc00020a5a0, {0x14ecda0, 0xc000702680}, 0xc000985320, 0xc0002828a0, 0x1c089a0, 0x0)
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/vendor/google.golang.org/grpc/server.go:1335 +0xdf3
google.golang.org/grpc.(*Server).handleStream(0xc00020a5a0, {0x14ecda0, 0xc000702680}, 0xc000985320, 0x0)
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/vendor/google.golang.org/grpc/server.go:1712 +0xa36
google.golang.org/grpc.(*Server).serveStreams.func1.1()
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/vendor/google.golang.org/grpc/server.go:947 +0xca
created by google.golang.org/grpc.(*Server).serveStreams.func1
        /tmp/guix-build-terraform-provider-consul-2.20.0.drv-0/source/source/vendor/google.golang.org/grpc/server.go:958 +0x15c

Error: The terraform-provider-consul plugin crashed!

There is already a PR open to potentially fix this if it could be prioritized