Bad validation on Global Secondary Indexes

[reconbot]

Describe the issue
I added a global secondary index in development using Dynalite and when I went to production I had issues because I wasn't always providing data for the hashkey in the GSI. However the value wasn't undefined but instead an empty string.

In development it didn't care, in production I got this error on put;

One or more parameter values are not valid. A value specified for a secondary index key is not supported. The AttributeValue for a key attribute cannot contain an empty string value. IndexName: gsi2, IndexKey: gsi2pk

Steps to reproduce
Steps to reproduce the behavior:

1. create a table with a global secondary index with string hash and sort keys
2. put a record that's got empty string values for those fields
3. Look to get an error, but get a successful put

Expected behavior
An error on put