wdlTools 0.17.16

• New value type V_ForcedNull is used to avoid an addition of an explicit null input to an Optional value. Previously,
  if a task was wrapped in a frag - in some cases Optionals without a value were forced to be an explicit null. This version
  fixes such behavior.
• Draft-2 parser. Previous behavior: if version is not declared, it is assumed to be draft-2, which has no formal
  input definitions for tasks and workflows. However, if a user forgets to declare the version while working on workflow
  of a version greater than draft-2 with formal definition of input, the parser fails to follow the syntax and throws
  the errors not related to the version or the formatting of the input section. Here this behavior is fixed: if version
  is not detected and draft-2 is assumed, we check if the input section is defined and throw an exception in this case.