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/server/transports/stdio_transport_test.rb

@@ -45,7 +45,7 @@ module MCP
             response = JSON.parse(output.string, symbolize_names: true)
             assert_equal("2.0", response[:jsonrpc])
             assert_equal("123", response[:id])
-            assert_empty(response[:result])
+            assert_equal({}, response[:result])
             refute(@transport.instance_variable_get(:@open))
           ensure
             $stdin = original_stdin