Set default for rpcError

[gsteinLTU]

Fix #1314

Is there something better than null we could use for "no RPC error"?

[gsteinLTU]

Now uses empty string instead of null

[brollb]

I just remembered that some of the early NetsBlox projects used the following check for RPC errors:

Although this isn't really ideal, we should probably stick to the current convention (avoiding breaking changes) and should use null for now.

I am going to revert back to your original commit and merge. Thanks!