Avoid logging error when <empty> geometry is included

[shonigmann]

This is a very small change to avoid an error message while using geometry in models. I simply added a check for the msgs::Geometry::EMPTY type with an empty statement to avoid writing an error message for "Unrecognized geometry type" in the default else statement.

geometry is listed as a valid option the SDF spec, and is already supported by GeometryFromSDF().