[shape.bzl] generate thrift impl as long as __thrift is set at all

Summary:
It can be useful to make empty structs as sentinel types which generate unit
structs in Rust.
Update the Rust code generator to generate thrift impls as long as `__thrift`
is non-`null`

Test Plan:
On D68505354
```
arc rust-check fbcode//metalos/bzl/service:service.shape-rust
```

Reviewed By: sergeyfd

Differential Revision: D68506178

fbshipit-source-id: 855e700ea36f4a1a2b63bd90d3b619e36c1db28d

antlir/bzl/shape2/ir2code.rs

@@ -161,6 +161,7 @@ where
     handlebars.register_helper("json", Box::new(json_helper));
     handlebars.register_helper("upper", Box::new(upper_helper));
     handlebars.register_helper("ident", Box::new(ident_helper));
+    handlebars.register_helper("is-null", Box::new(is_null_helper));
 
     let handlebars = <Module as RegisterTemplates<T>>::register_templates(handlebars, templates)
         .context("When setting up handlebars for Module")?;
@@ -180,6 +181,7 @@ where
 handlebars_helper!(upper_helper: |x: String| x.to_uppercase());
 handlebars_helper!(ident_helper: |x: String| x.replace('-', "_"));
 handlebars_helper!(can_default: |x: BTreeMap<FieldName, Arc<Field>>| x.values().all(|v| !v.required || v.default_value.is_some()));
+handlebars_helper!(is_null_helper: |x: Value| x.is_null());
 
 trait ModuleExt<T>
 where

antlir/bzl/shape2/templates/struct.rs.handlebars

@@ -39,7 +39,7 @@ impl {{name}} {
     {{/each}}
 }
 
-{{#if thrift_fields}}
+{{#unless (is-null thrift_fields)}}
 impl ::fbthrift::ttype::GetTType for {{name}} {
     const TTYPE: ::fbthrift::ttype::TType = ::fbthrift::ttype::TType::Struct;
 }
@@ -115,7 +115,7 @@ where
         })
     }
 }
-{{/if}}
+{{/unless}}
 
 {{#if (can-default fields)}}
 impl Default for {{name}} {