Remove `<empty>` from `<geometry>`

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/174)) by Shane Loretz (Bitbucket: [Shane Loretz](https://bitbucket.org/%7B656e3311-aad9-45a1-aaf7-b0ee0e84b287%7D/), GitHub: [sloretz](https://github.com/sloretz)).**

----------------------------------------
The 1.6 spec allows an emtpy geometry via the tag ``. XML already has mechanisms for empty tags though.

```

```

The `` tag could be eliminated by making these legal SDF
```

```

or equivalently
```

```

Start with the SDF 1.6 specification and the archived issue linked in the report. Determine how empty geometry is currently represented and identify the parser and specification tests that define valid geometry syntax; done means empty geometry forms are accepted without requiring the tag.

Written by the indexing model from the issue text.