Fix AugmentationSchemaProxy.getDataChildByName()

This implementation breaks the API contract specified by
AugmentationDefinition by throwing IllegalArgumentException where it
should just return null.

Drive-by: fix a typo in AugmentationSchema

Change-Id: I762ce3c02ba776d8a025ccd5bcdba82a6c1579ad
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Fix AugmentationSchemaProxy.getDataChildByName()"