Runtime error with default string = ""

[Ben-Habermeyer]

Hi DXCompiler team,

I recently ran into a runtime IOerror No value found for I O spec field caller_to_emulate when calling a workflow with a String input that is "" by default, using dxCompiler 2.10.2

For example

workflow1 {
  input {
    Int req_input
    Int random_seed = -1
    String caller_to_emulate = ""
  }
... 
}

# workflow 2 calls workflow 1, but does not provide caller to emulate
workflow2 {
...
  call workflow1 {
    input:
        req_input=1
  }
}

I haven't found anything in the wdl spec as to why an empty string would not be accepted as a default. https://github.com/openwdl/wdl/blob/main/versions/1.1/SPEC.md#concatenation-of-optional-values could you investigate this one?