Fix OBJExporter error on THREE.Line<BufferGeometry>

[Hudi304]

Why

I negated an if so that if a THREE.Line object that is passed to the exporter and it has a geometry of type BufferGeometry the exporter does not throw an error.

Issue #412

What

I negated the condition.

Checklist

• [ ] Documentation updated (I don't think it's relevant)
• [ ] Storybook entry added (apologies I can not find it)
• [x] Ready to be merged

[codesandbox-ci[bot]]

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