Fix bug #2790076; running a procedure that has only empty body lines no longer crashes. This is a variant of a bug that was fixed a long time ago, but it appears as if the fix only works if the procedure is run once. After digging into the code, I think this is because the evaluator assumes that that if is_tree(node) is non-zero, then treepair__tree(node) will be non-NIL. This assumption is violated when a body list consisted of entirely empty lines was treeified. The previous (inadequate) fix was to add some logic to remove this assumption from the evaluator. The new fix is to simply not flag these degenerate functions as being treeified. This causes the evaluator to fail-out, instead of trying to run a procedure with no meaningful body. This seems to work, but all changes to the evaluator are risky and can cause subtle regressions.
