Fix and clean up BufferGeometry/Attribute serialization

[takahirox]

Changes

• Add morphAttribute to serialized buffer geometry data only when geometry.morphAttribute isn't empty. Fix travis error. See Add morphAttributes serialization support to BufferGeometry #11596 (comment)
• Serialize non-morph attribute name
• Add name to serialized buffer attribute data only when attribute.name isn't empty
• Minor clean up
• Update BufferGeometry.toJSON() test

[mrdoob]

Thanks!