In the spec for GetStackTrace, it says it will return an error if
start_depth >= stackDepth.

This means that for an empty stack (stackDepth ==0), it is impossible to
query the stack trace.

The RI seems to have a special exemption for start_depth == 0 and
stackDepth ==0 to allow the "default" case (start_depth == 0) to work on
empty stacks. Perhaps a clarification should be added to the spec for this
case (basically, always allow start_depth == 0, with no check against
stackDepth).