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

References:

* https://github.com/hashicorp/terraform/issues/14418
* v0.9.5 (original bug report): a59ee0b30e/helper/schema/field_writer_map.go (L311)
* v0.11.12 (Terraform AWS Provider discovery): 057286e522/helper/schema/field_writer_map.go (L343)

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

```go
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/terraform@v0.11.12/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
```

helper/schema/field_writer_map.go

@@ -341,6 +341,11 @@ func (w *MapFieldWriter) setSet(
 	// problems when the old data isn't wiped first.
 	w.clearTree(addr)
 
+	if value.(*Set) == nil {
+		w.result[k+".#"] = "0"
+		return nil
+	}
+
 	for code, elem := range value.(*Set).m {
 		if err := w.set(append(addrCopy, code), elem); err != nil {
 			return err

helper/schema/field_writer_map_test.go

@@ -203,6 +203,15 @@ func TestMapFieldWriter(t *testing.T) {
 			},
 		},
 
+		"set typed nil": {
+			[]string{"set"},
+			func() *Set { return nil }(),
+			false,
+			map[string]string{
+				"set.#": "0",
+			},
+		},
+
 		"set resource": {
 			[]string{"setDeep"},
 			[]interface{}{