Inserting nothing in Pyret gives a parse error

[schanzer]

[From https://github.com/bootstrapworld/codemirror-blocks/issues/248]
Start with the code

f(x)

Now double-click on the drop target to one side of the x, and insert "" or " ".

This should be a no-op: you haven't inserted anything. Instead, it's a parse error. What happens is:

1. A copy of the node f(x) is made.
2. A new child is inserted into this node. This child is a fake node that prints itself out as " ", since that's what you inserted.
3. The copied node is pretty-printed, producing f(x, ).
4. The edit is thus taken to be: f(x) => f(x, ).
5. Hence, parse error.

---

To fix this, I think we should add a check to see if text is purely whitespace in src/edits/performEdits/edit_insert(). If it is, skip the insert because nothing is being inserted.

TODO:

• Fix this.
• Think: will a similar problem plague any other kind of edit? If so, fix that too.