Empty attributes in a <plugin/> tag are not printed by Element::ToString unless they are in a custom XML namespace

[scpeters]

Environment

• OS Version: macOS: 13.6.6-x86_64
• Source or binary build? source build of sdf14 as of c9779cb7f19bac92b512e213b7fd50dbcf2c8c8d (just after merge of https://github.com/gazebosim/sdformat/pull/1407)

Description

• Expected behavior: as of #1407, attributes within a <plugin/> set to an empty string "" can now be parsed without emitting errors, so the empty attributes should be included in the string generated by Element::ToString.
• Actual behavior: Only empty attributes in a custom XML namespace will be printed by Element::ToString.

Steps to reproduce

1. Apply the following patch (from https://github.com/gazebosim/sdformat/pull/1407#issuecomment-2088798546):

diff --git a/test/integration/plugin_attribute.cc b/test/integration/plugin_attribute.cc
index 7f4f9f4c..b66c9da1 100644
--- a/test/integration/plugin_attribute.cc
+++ b/test/integration/plugin_attribute.cc
@@ -65,4 +65,9 @@ TEST(PluginAttribute, ParseAttributes)
     EXPECT_EQ(value->Get<std::string>(""),
               std::string("value"));
   }
+  {
+    auto pluginToString = plugin->ToString("");
+    EXPECT_NE(std::string::npos, pluginToString.find("empty_attribute=''"))
+      << pluginToString;
+  }
 }

2. Build and run INTEGRATION_plugin_attribute test
3. Observe test failure

[ RUN      ] PluginAttribute.ParseAttributes
/Users/scpeters/ws/dome/src/sdformat/test/integration/plugin_attribute.cc:70: Failure
Expected: (std::string::npos) != (pluginToString.find("empty_attribute=''")), actual: 18446744073709551615 vs 18446744073709551615
<plugin name='example' filename='libexample.so'>
  <user attribute='attribute'/>
  <value attribute='attribute'>value</value>
</plugin>

[  FAILED  ] PluginAttribute.ParseAttributes (56 ms)

Output

[scpeters]

As I noted in https://github.com/gazebosim/sdformat/pull/1407#issuecomment-2088798546, I don't have a good idea for how to fix this. I'm going to close this as "won't fix", but feel free to comment here if you have an idea and we can reconsider it.