Fix: nil deref bug for ruleset response (#2762)

Example panic:

Stack trace from the terraform-provider-github_v6.6.0 plugin:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xcac607]

goroutine 4723 [running]:
github.com/integrations/terraform-provider-github/v6/github.resourceGithubRepositoryRulesetRead(0xc000b0d300, {0xd352c0?, 0xc00089ea00?})
	github.com/integrations/terraform-provider-github/v6/github/resource_github_repository_ruleset.go:580 +0x567
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).read(0x107d470?, {0x107d470?, 0xc000a40ff0?}, 0xd?, {0xd352c0?, 0xc00089ea00?})
	github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/resource.go:811 +0x15f
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).RefreshWithoutUpgrade(0xc0003ed7a0, {0x107d470, 0xc000a40ff0}, 0xc000b1b1e0, {0xd352c0, 0xc00089ea00})
	github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/resource.go:1117 +0x529
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*GRPCProviderServer).ReadResource(0xc00036c678, {0x107d470?, 0xc000a40cf0?}, 0xc0006dea40)
	github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/grpc_provider.go:708 +0x6c5
github.com/hashicorp/terraform-plugin-go/tfprotov5/tf5server.(*server).ReadResource(0xc00026d900, {0x107d470?, 0xc000a404b0?}, 0xc00025c3f0)
	github.com/hashicorp/[email protected]/tfprotov5/tf5server/server.go:783 +0x309
github.com/hashicorp/terraform-plugin-go/tfprotov5/internal/tfplugin5._Provider_ReadResource_Handler({0xee16c0, 0xc00026d900}, {0x107d470, 0xc000a404b0}, 0xc000b0ce00, 0x0)
	github.com/hashicorp/[email protected]/tfprotov5/internal/tfplugin5/tfplugin5_grpc.pb.go:482 +0x1a6
google.golang.org/grpc.(*Server).processUnaryRPC(0xc000197000, {0x107d470, 0xc000a40420}, {0x1081d68, 0xc00027c300}, 0xc0009a1200, 0xc00045aab0, 0x1719528, 0x0)
	google.golang.org/[email protected]/server.go:1369 +0xdf8
google.golang.org/grpc.(*Server).handleStream(0xc000197000, {0x1081d68, 0xc00027c300}, 0xc0009a1200)
	google.golang.org/[email protected]/server.go:1780 +0xe8b
google.golang.org/grpc.(*Server).serveStreams.func2.1()
	google.golang.org/[email protected]/server.go:1019 +0x8b
created by google.golang.org/grpc.(*Server).serveStreams.func2 in goroutine 12
	google.golang.org/[email protected]/server.go:1030 +0x125

Error: The terraform-provider-github_v6.6.0 plugin crashed!

Co-authored-by: Nick Floyd <[email protected]>

Author: eldad

github/resource_github_repository_ruleset.go

@@ -630,6 +630,13 @@ func resourceGithubRepositoryRulesetRead(d *schema.ResourceData, meta interface{
 		}
 	}
 
+	if ruleset == nil {
+		log.Printf("[INFO] Removing ruleset %s/%s: %d from state because it no longer exists in GitHub (empty response)",
+			owner, repoName, rulesetID)
+		d.SetId("")
+		return nil
+	}
+
 	d.Set("etag", resp.Header.Get("ETag"))
 	d.Set("name", ruleset.Name)
 	d.Set("target", ruleset.GetTarget())