Fix ResolveReferences for Empty Sidesites

[tomstewart89]

Hello! This PR should fix the issue I raised over here: #3119

The Problem

• Define a model containing a spatial tendon with a wrapping geom that doesn't specify a sidesite
• Attach that model to another using mjs_attach and specify a prefix and/or suffix
• Execution finds its way to mjCTendon::ResolveReferences where the sidesite is named prefix + "" + suffix
• mjCWrap::ResolveReferences tries to find the site prefix + "" + suffix, fails and throws.
• That exception is caught in mjCModel::CopyList and the tendon is skipped due to this continue statement

The Fix

Don't apply the prefix and suffix when a spatial tendon's sidesite is empty. That'll let mjCTendon::ResolveReferences know that it need not search for the site.

To add some test coverage for this, I also added a spatial tendon to the xml_child used by MujocoTest.AttachSame, MujocoTest.AttachDifferent and MujocoTest.AttachFrame in user_api_test.cc. These tests should all fail without the change added to user_objects.cc

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[tomstewart89]

This fix is now implemented with 6ec808e so I'll go ahead and close this.