Empty string should be a valid value for default value of defineVariable but when trying to use it I get the following error:

edu.illinois.ncsa.daffodil.processors.ParseError: Parse Error: Expression evaluation failed. Details: javax.xml.xpath.XPathExpressionException: Expression result is not a node

From the spec - To set the default value to "" (empty string), the defaultValue attribute syntax must be used, or the expression

must be used as the element value.

See tests:
ngf-dfdl-2783:Variables_r1-test_emptyDefault
ngf-dfdl-2784:Variables_r2-test_emptyDefault2

In files:
daffodil-test/src/test/resources/edu/illinois/ncsa/daffodil/section07/variables/variables.tdml
daffodil-test/src/test/scala-debug/edu/illinois/ncsa/daffodil/section07/variables/TestVariablesDebug.scala