fix: avoid panic when calculating label for block

[tim775]

I wasn't able to reproduce but we got a runtime error: invalid memory address or nil pointer dereference.

Stack trace is:

   /panic.go:770                                    panic()
   internal/hcl/block.go:1276                       (*Block).Label()
   internal/providers/terraform/hcl_provider.go:412 (*HCLProvider).marshalModule.func1()
   sort/zsortfunc.go:12                             insertionSort_func()
   sort/zsortfunc.go:343                            stable_func()
   sort/slice.go:44                                 SliceStable()
   internal/providers/terraform/hcl_provider.go:411 (*HCLProvider).marshalModule()
   internal/providers/terraform/hcl_provider.go:440 (*HCLProvider).marshalModule()
   internal/providers/terraform/hcl_provider.go:440 (*HCLProvider).marshalModule()
   internal/providers/terraform/hcl_provider.go:440 (*HCLProvider).marshalModule()
   internal/providers/terraform/hcl_provider.go:440 (*HCLProvider).marshalModule()
   internal/providers/terraform/hcl_provider.go:440 (*HCLProvider).marshalModule()
   internal/providers/terraform/hcl_provider.go:382 (*HCLProvider).modulesToPlanJSON()
   internal/providers/terraform/hcl_provider.go:302 (*HCLProvider).LoadPlanJSON()
   internal/providers/terraform/hcl_provider.go:215 (*HCLProvider).LoadResources()
   main/run.go:427                                  (*parallelRunner).runProvider()
   main/run.go:282                                  (*parallelRunner).run.func1()
   golang.org/x/sync/errgroup/errgroup.go:75        (*Group).Go.func1()