[mlir] Reduce warnings for bad assertion in generated code

[clementval]

Summary

When the operation has no attributes, the generated assertion is
always false and triggers lots of warnings in the build.

warning: comparison of unsigned expression < 0 is always false

Just return a StringAttr when there is no attribute.

mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp

void OpEmitter::genAttrNameGetters() {
   }
   {
     auto *method = opClass.addStaticInlineMethod<Method::Private>(
         "::mlir::StringAttr", "getAttributeNameForIndex",
         MethodParameter("::mlir::OperationName", "name"),
         MethodParameter("unsigned", "index"));
     ERROR_IF_PRUNED(method, "getAttributeNameForIndex", op);
 
+    if (attributes.empty()) {
+      method->body() << "  return {};";
+    } else {
       const char *const getAttrName = R"(
   assert(index < {0} && "invalid attribute index");
   assert(name.getStringRef() == getOperationName() && "invalid operation name");
   return name.getAttributeNames()[index];
 )";
       method->body() << formatv(getAttrName, attributes.size());
     }
+  }
 
   // Generate the <attr>AttrName methods, that expose the attribute names to
   // users.
   const char *attrNameMethodBody = "  return getAttributeNameForIndex({0});";
   for (auto [index, attr] :
        llvm::enumerate(llvm::make_first_range(attributes))) {
     std::string name = op.getGetterName(attr);
     std::string methodName = name + "AttrName";