Allow nil return, and fix typing

lib/ruby_indexer/lib/ruby_indexer/index.rb

@@ -619,13 +619,12 @@ def existing_or_new_singleton_class(name)
     sig do
       type_parameters(:T).params(
         path: String,
-        type: T.nilable(T::Class[T.all(T.type_parameter(:T), Entry)]),
-      ).returns(T::Array[T.type_parameter(:T)])
+        type: T::Class[T.all(T.type_parameter(:T), Entry)],
+      ).returns(T.nilable(T::Array[T.type_parameter(:T)]))
     end
-    def entries_for(path, type = nil)
-      type = Entry if type.nil?
-
-      Array(@files_to_entries[path]).grep(type)
+    def entries_for(path, type)
+      entries = @files_to_entries[path]
+      entries&.grep(type)
     end
 
     private

lib/ruby_indexer/test/index_test.rb

@@ -1853,15 +1853,15 @@ def self.my_singleton_def; end
         end
       RUBY
 
-      entries = @index.entries_for("/fake/path/foo.rb")
+      entries = @index.entries_for("/fake/path/foo.rb", Entry)
       assert_equal(["Foo", "Bar", "my_def", "Bar::<Class:Bar>", "my_singleton_def"], entries.map(&:name))
 
       entries = @index.entries_for("/fake/path/foo.rb", RubyIndexer::Entry::Namespace)
       assert_equal(["Foo", "Bar", "Bar::<Class:Bar>"], entries.map(&:name))
     end
 
     def test_entries_for_returns_nil_if_no_matches
-      assert_empty(@index.entries_for("non_existing_file.rb"))
+      assert_nil(@index.entries_for("non_existing_file.rb", Entry::Namespace))
     end
   end
 end