internal/fwserver: Prevent PlanResourceChange panic during changed path logging (#797)

Reference: #783

Prior to the logic update:

```
--- FAIL: TestServerPlanResourceChange (0.01s)
    --- FAIL: TestServerPlanResourceChange/update-set-default-values (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=0x2 addr=0x18 pc=0x1043b0484]

goroutine 32 [running]:
testing.tRunner.func1.2({0x1048d4e60, 0x104c08f90})
	/opt/homebrew/Cellar/go/1.20.5/libexec/src/testing/testing.go:1526 +0x1c8
testing.tRunner.func1()
	/opt/homebrew/Cellar/go/1.20.5/libexec/src/testing/testing.go:1529 +0x384
panic({0x1048d4e60, 0x104c08f90})
	/opt/homebrew/Cellar/go/1.20.5/libexec/src/runtime/panic.go:884 +0x204
github.com/hashicorp/terraform-plugin-framework/internal/fwserver.(*Server).PlanResourceChange(0x140001f4a00, {0x10494ddb0, 0x14000098008}, 0x140001eeeb0, 0x14000592540)
	/Users/bflad/src/github.com/hashicorp/terraform-plugin-framework/internal/fwserver/server_planresourcechange.go:219 +0x19e4
github.com/hashicorp/terraform-plugin-framework/internal/fwserver_test.TestServerPlanResourceChange.func37(0x14000231860)
	/Users/bflad/src/github.com/hashicorp/terraform-plugin-framework/internal/fwserver/server_planresourcechange_test.go:5849 +0x70
testing.tRunner(0x14000231860, 0x140000ccde0)
	/opt/homebrew/Cellar/go/1.20.5/libexec/src/testing/testing.go:1576 +0x10c
created by testing.(*T).Run
	/opt/homebrew/Cellar/go/1.20.5/libexec/src/testing/testing.go:1629 +0x368
FAIL	github.com/hashicorp/terraform-plugin-framework/internal/fwserver	0.457s
```

This change verifies and handles the more critical bug fix of the panic report. The handling of defaults for set nested attributes will require further logic updates, which will be handled in a future change.

Author: bflad

.changes/unreleased/BUG FIXES-20230711-161747.yaml

@@ -0,0 +1,6 @@
+kind: BUG FIXES
+body: 'resource: Prevented panic during planning caused by `SetNestedAttribute` with
+  nested attribute `Default` and multiple configured elements'
+time: 2023-07-11T16:17:47.990142-04:00
+custom:
+  Issue: "783"

internal/fwschemadata/data_get_at_path_test.go

@@ -34,6 +34,40 @@ func TestDataGetAtPath(t *testing.T) {
 		expected      any
 		expectedDiags diag.Diagnostics
 	}{
+		"invalid-path": {
+			data: fwschemadata.Data{
+				Schema: testschema.Schema{
+					Attributes: map[string]fwschema.Attribute{
+						"test": testschema.Attribute{
+							Optional: true,
+							Type:     types.StringType,
+						},
+					},
+				},
+				TerraformValue: tftypes.NewValue(
+					tftypes.Object{
+						AttributeTypes: map[string]tftypes.Type{
+							"test": tftypes.String,
+						},
+					},
+					map[string]tftypes.Value{
+						"test": tftypes.NewValue(tftypes.String, "test-value"),
+					},
+				),
+			},
+			path:     path.Root("not-test"),
+			target:   new(string),
+			expected: new(string),
+			expectedDiags: diag.Diagnostics{
+				diag.NewAttributeErrorDiagnostic(
+					path.Root("not-test"),
+					"Data Read Error",
+					"An unexpected error was encountered trying to retrieve type information at a given path. "+
+						"This is always an error in the provider. Please report the following to the provider developer:\n\n"+
+						"Error: AttributeName(\"not-test\") still remains in the path: could not find attribute or block \"not-test\" in schema",
+				),
+			},
+		},
 		"invalid-target": {
 			data: fwschemadata.Data{
 				Schema: testschema.Schema{
@@ -104,6 +138,29 @@ func TestDataGetAtPath(t *testing.T) {
 				),
 			},
 		},
+		"TerraformValue-null": {
+			data: fwschemadata.Data{
+				Schema: testschema.Schema{
+					Attributes: map[string]fwschema.Attribute{
+						"test": testschema.Attribute{
+							Optional: true,
+							Type:     types.StringType,
+						},
+					},
+				},
+				TerraformValue: tftypes.NewValue(
+					tftypes.Object{
+						AttributeTypes: map[string]tftypes.Type{
+							"test": tftypes.String,
+						},
+					},
+					nil,
+				),
+			},
+			path:     path.Root("test"),
+			target:   new(types.String),
+			expected: pointer(types.StringNull()),
+		},
 		"AttrTypeWithValidateError": {
 			data: fwschemadata.Data{
 				Schema: testschema.Schema{

internal/fwserver/server_planresourcechange.go

@@ -207,6 +207,14 @@ func (s *Server) PlanResourceChange(ctx context.Context, req *PlanResourceChange
 				_ = resp.PlannedState.GetAttribute(ctx, p, &plannedState)
 				_ = req.PriorState.GetAttribute(ctx, p, &priorState)
 
+				// Due to ignoring diagnostics, the value may not be populated.
+				// Prevent the panic and show the path as changed.
+				if plannedState == nil {
+					changedPaths.Append(p)
+
+					continue
+				}
+
 				if plannedState.Equal(priorState) {
 					continue
 				}