Replace assert_empty with assert_equal using explicit literals

Due to overly aggressive detection by RuboCop Minitest, the distinction between `[]` and `{}` was uniformly replaced with `assert_empty`,
making it impossible to differentiate between them.
As a result of https://github.com/rubocop/rubocop-minitest/issues/344, where the `Minitest/AssertEmptyLiteral` cop was disabled,
the return value has been expressed using explicit literal forms.

test/mcp/tool/input_schema_test.rb

@@ -33,7 +33,7 @@ module MCP
 
       test "missing_required_arguments returns an empty array if no required arguments are missing" do
         input_schema = InputSchema.new(properties: { message: { type: "string" } }, required: ["message"])
-        assert_empty input_schema.missing_required_arguments({ message: "Hello, world!" })
+        assert_equal([], input_schema.missing_required_arguments({ message: "Hello, world!" }))
       end
 
       test "valid schema initialization" do