helper/schema: Prevent setSet() panic with typed nil

[bflad]

Closes #14418

When creating flatten functions in Terraform Providers that return *schema.Set, it is possible to return a typed nil, e.g.

func flattenHeaders(h *cloudfront.Headers) *schema.Set {
	if h.Items != nil {
		return schema.NewSet(schema.HashString, flattenStringList(h.Items))
	}
	return nil
}

This previously could cause a panic, e.g.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x1881911]

goroutine 1325 [running]:
github.com/hashicorp/terraform/helper/schema.(*MapFieldWriter).setSet(0xc00054bf00, 0xc00073efa0, 0x5, 0x5, 0x5828140, 0x0, 0xc0002cea50, 0xc000e996a8, 0xc001026e40)
	/Users/bflad/go/pkg/mod/github.com/hashicorp/[email protected]/helper/schema/field_writer_map.go:343 +0x211

Here we catch the typed nil and return an empty list flatmap result instead.

Unit testing result prior to code update:

--- FAIL: TestMapFieldWriter (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
  panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x1777cdc]

goroutine 913 [running]:
testing.tRunner.func1(0xc00045b800)
  /usr/local/Cellar/go/1.12.1/libexec/src/testing/testing.go:830 +0x392
panic(0x192cf20, 0x2267ca0)
  /usr/local/Cellar/go/1.12.1/libexec/src/runtime/panic.go:522 +0x1b5
github.com/hashicorp/terraform/helper/schema.(*MapFieldWriter).setSet(0xc0004648a0, 0xc0004408d0, 0x1, 0x1, 0x19e3de0, 0x0, 0xc00045c600, 0x30, 0x19e0080)
  /Users/bflad/src/github.com/hashicorp/terraform/helper/schema/field_writer_map.go:344 +0x68c
github.com/hashicorp/terraform/helper/schema.(*MapFieldWriter).set(0xc0004648a0, 0xc0004408d0, 0x1, 0x1, 0x19e3de0, 0x0, 0x1, 0x18)
  /Users/bflad/src/github.com/hashicorp/terraform/helper/schema/field_writer_map.go:107 +0x28b
github.com/hashicorp/terraform/helper/schema.(*MapFieldWriter).WriteField(0xc0004648a0, 0xc0004408d0, 0x1, 0x1, 0x19e3de0, 0x0, 0x0, 0x0)
  /Users/bflad/src/github.com/hashicorp/terraform/helper/schema/field_writer_map.go:89 +0x504
github.com/hashicorp/terraform/helper/schema.TestMapFieldWriter(0xc00045b800)
  /Users/bflad/src/github.com/hashicorp/terraform/helper/schema/field_writer_map_test.go:337 +0x2ddd
testing.tRunner(0xc00045b800, 0x1a44f90)
  /usr/local/Cellar/go/1.12.1/libexec/src/testing/testing.go:865 +0xc0
created by testing.(*T).Run
  /usr/local/Cellar/go/1.12.1/libexec/src/testing/testing.go:916 +0x35a

References:

• v0.9.5 (original bug report):

  terraform/helper/schema/field_writer_map.go

  Line 311 in a59ee0b

  for code, elem := range value.(*Set).m {

• v0.11.12 (Terraform AWS Provider discovery):

  terraform/helper/schema/field_writer_map.go

  Line 343 in 057286e

  for code, elem := range value.(*Set).m {

[ghost]

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