GET request providing only partitionKey on a table with a defined sortKey results in error

[bwermes]

Describe the bug
When performing a GET request providing only the partitionKey against a table that also contains a sortKey results in an error:
please provide the sort key for the attribute name

My table contains

To Reproduce
Steps to reproduce the behavior:

1. Create a table with a PartitionKey and a SortKey:
   `@Model({tableName: 'Sample'})
   export class className{
   @PartitionKey()
   pk: string

   @SortKey()
   sk: string

   name: string;

   public constructor(init?: Partial) {
   Object.assign(this, init);
   }
   }`

2. Perform a GET request against that table:
   const sample = await new DynamoStore(Sample).get('myPartitionKey');

3. Observe the error

Expected behavior
The GET method provides for the SortKey as optional. Other Dynamo libraries allow you to query a single record by PartitionKey only, returning the first record if multiple exist.