hcl2template: don't error on empty bucket slug

When a user defines a `hcp_packer_registry` block in their `build`
without a `bucket_name`, but they define it in their environment, Packer
should not report the bucket_name being wrong.

hcl2template/testdata/hcp_par/empty_bucket.pkr.hcl

@@ -0,0 +1,11 @@
+source "null" "test" {
+  communicator = "none"
+}
+
+build {
+  name = "bucket-slug"
+  hcp_packer_registry {
+  }
+
+  sources = ["null.test"]
+}

hcl2template/types.build.hcp_packer_registry.go

@@ -54,7 +54,12 @@ func (p *Parser) decodeHCPRegistry(block *hcl.Block, cfg *PackerConfig) (*HCPPac
 		return nil, diags
 	}
 
-	if !bucketNameRegexp.MatchString(b.Slug) {
+	// No need to check the bucket name here if it's empty, since it can
+	// be set through the `HCP_PACKER_BUCKET_NAME` environment var.
+	//
+	// If both are unset, creating the build on HCP Packer will fail, and
+	// so will the packer build command.
+	if b.Slug != "" && !bucketNameRegexp.MatchString(b.Slug) {
 		diags = diags.Append(&hcl.Diagnostic{
 			Severity: hcl.DiagError,
 			Summary:  fmt.Sprintf("%s.bucket_name can only contain between 3 and 36 ASCII letters, numbers and hyphens", buildHCPPackerRegistryLabel),

hcl2template/types.build.hcp_packer_registry_test.go

@@ -20,6 +20,47 @@ func Test_ParseHCPPackerRegistryBlock(t *testing.T) {
 	defaultParser := getBasicParser()
 
 	tests := []parseTest{
+		{"bucket_name left empty",
+			defaultParser,
+			parseTestArgs{"testdata/hcp_par/empty_bucket.pkr.hcl", nil, nil},
+			&PackerConfig{
+				CorePackerVersionString: lockedVersion,
+				Basedir:                 filepath.Join("testdata", "hcp_par"),
+				Sources: map[SourceRef]SourceBlock{
+					refNull: {
+						Type: "null",
+						Name: "test",
+						block: &hcl.Block{
+							Type: "source",
+						},
+					},
+				},
+				Builds: Builds{
+					{
+						Name:              "bucket-slug",
+						HCPPackerRegistry: &HCPPackerRegistryBlock{Slug: ""},
+						Sources: []SourceUseBlock{
+							{
+								SourceRef: refNull,
+							},
+						},
+					},
+				},
+			},
+			false, false,
+			[]packersdk.Build{
+				&packer.CoreBuild{
+					BuildName:      "bucket-slug",
+					Type:           "null.test",
+					Builder:        &null.Builder{},
+					Provisioners:   []packer.CoreBuildProvisioner{},
+					PostProcessors: [][]packer.CoreBuildPostProcessor{},
+					Prepared:       true,
+					BuilderType:    "null",
+				},
+			},
+			false,
+		},
 		{"bucket_name as variable",
 			defaultParser,
 			parseTestArgs{"testdata/hcp_par/variable-for-bucket_name.pkr.hcl", nil, nil},