revert: revert omit nil and empty string slices (#3492)

Fields with empty slices (e.g. disabled_rustdoc_warnings: []) are
meaningful to generator to signal differences from default. context:
#3484 (comment)

Added comment to call out this behavior.

Test out locally, no diff shown: 
In the librarian repo
`git checkout revert-3434-pr3`
`go install ./cmd/librarian`
`go install ./tool/cmd/migrate`
Run migration and generation from the google-cloud-rust repo:
`migrate .`
`librarian generate common`

Reverts #3434
Fix #3484

Author: zhumin8

internal/config/language.go

@@ -14,6 +14,8 @@
 
 package config
 
+import "github.com/googleapis/librarian/internal/yaml"
+
 // GoModule represents the Go-specific configuration for a library.
 type GoModule struct {
 	DeleteGenerationOutputPaths []string `yaml:"delete_generation_output_paths,omitempty"`
@@ -46,8 +48,8 @@ type RustDefault struct {
 // Each module specifies what proto source to use, which template to apply,
 // and where to output the generated code.
 type RustModule struct {
-	// DisabledRustdocWarnings is a list of rustdoc warnings to disable.
-	DisabledRustdocWarnings []string `yaml:"disabled_rustdoc_warnings,omitempty"`
+	// DisabledRustdocWarnings specifies rustdoc lints to disable. An empty slice explicitly enables all warnings.
+	DisabledRustdocWarnings yaml.StringSlice `yaml:"disabled_rustdoc_warnings,omitempty"`
 
 	// DocumentationOverrides contains overrides for element documentation.
 	DocumentationOverrides []RustDocumentationOverride `yaml:"documentation_overrides,omitempty"`

internal/yaml/yaml.go

@@ -23,6 +23,23 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
+// StringSlice is a custom slice of strings that allows for fine-grained control
+// over YAML marshaling when used with the 'omitempty' tag.
+//
+// By implementing the yaml.IsZeroer interface, it ensures that:
+//  1. A nil slice is considered "zero" and is omitted from the output.
+//  2. An empty but initialized slice (e.g., []string{}) is NOT considered "zero"
+//     and is explicitly marshaled as an empty YAML sequence ([]).
+type StringSlice []string
+
+// IsZero implements the yaml.IsZeroer interface, which determines whether a
+// field should be considered "empty" when the 'omitempty' struct tag is used.
+func (s StringSlice) IsZero() bool {
+	// return true ONLY if nil (omit field)
+	// return false if empty slice (keep field)
+	return s == nil
+}
+
 // Unmarshal parses YAML data into a value of type T.
 func Unmarshal[T any](data []byte) (*T, error) {
 	var v T

internal/yaml/yaml_test.go

@@ -87,3 +87,19 @@ func TestWriteError(t *testing.T) {
 		t.Error("Write() expected error for invalid path")
 	}
 }
+
+func TestStringSlice_EmptySlice(t *testing.T) {
+	strSlice := StringSlice{}
+	got := strSlice.IsZero()
+	if diff := cmp.Diff(false, got); diff != "" {
+		t.Errorf("mismatch (-want +got):\n%s", diff)
+	}
+}
+
+func TestStringSlice_NilSlice(t *testing.T) {
+	var strSlice StringSlice
+	got := strSlice.IsZero()
+	if diff := cmp.Diff(true, got); diff != "" {
+		t.Errorf("mismatch (-want +got):\n%s", diff)
+	}
+}