According to Spec item 3.1. Section 14.5. Changes to property hiddenGroupRef:

"Empty string is not an allowed value"

Though the TDML runner complains if you use the empty string for the hiddenGroupRef property, when tdml validation is off the data parses and no error is thrown.

See test hiddenGroupEmpty in the following files:
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1. modified: daffodil-test/src/test/resources/edu/illinois/ncsa/daffodil/section14/sequence_groups/SequenceGroup.tdml
2. modified: daffodil-test/src/test/scala-debug/edu/illinois/ncsa/daffodil/section14/sequence_groups/TestSequenceGroupsDebug.scala
3. modified: daffodil-test/src/test/scala/edu/illinois/ncsa/daffodil/section14/sequence_groups/TestSequenceGroups.scala