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/icon_test.rb

@@ -23,7 +23,7 @@ module MCP
       assert_nil(icon.src)
       assert_nil(icon.theme)
 
-      assert_empty(icon.to_h)
+      assert_equal({}, icon.to_h)
     end
 
     def test_valid_theme_for_light