How to allow Optional Key in API Gateway mapping template - json

Question

I have an API gateway that scans a DynamoDB table. I want to pass LastEvaluatedKey in my request body; if I pass a LastEvaluatedKey then the everything works and I get a response with the expected data - so I'm half way there.
But, of course, the first time I send a request to the API LastEvaluatedKey will not exist, so the ExclusiveStartKey in the mapping template that expects a LastEvaluatedKey must be optional. I've tried a few different ways to get this to be optional but nothing has worked so far. Here is what I have:
#set($hasName = $input.json('$.Name'))
{
"TableName": "MyTable",
#if($hasName && $hasName.length() != 0)
"ExclusiveStartKey": {
"Name": {
"S": $input.json('$.Name')
},
"Date": {
"S": $input.json('$.Date')
}
},#end
"FilterExpression": "begins_with(#dt, :tdt)",
"ExpressionAttributeNames": {
"#dt": "Date"
},
"ExpressionAttributeValues": {
":tdt": {
"S": "$input.params('date')"
}
}
}
As I say, the above works when I do pass a LastEvaluatedKey in my request body, but when I do not I get the error:
{
"__type": "com.amazon.coral.validate#ValidationException",
"message": "The provided starting key is invalid: One or more parameter values were invalid: An AttributeValue may not contain an empty string"
}
...it's still expected the LastEvaluatedKey
I've also tried wrapping Name and Date inside if #if but with no luck at all. I've taken inspiration from other answers such as: this and this, but no luck.

Answer 1

In my example cursor is the LastEvaluatedKey:
#set($hasCursor = $input.params('cursor') != "")
{
"TableName": "my-table"
#if($hasCursor) "ExclusiveStartKey":$util.base64Decode("$input.params('cursor')")
#end
}
It just checks if cursor is passed as a query param (like /my/api?cursor=myencodedlastevaluatedkey), and only adds ExclusiveStartKey to the request if it is.

Answer 2

If you know beforehand that both keys are string type, you can do:
#set($Name = $input.params('Name'))
#set($Date = $input.params('Date'))
{
#if($Name != '' && $Date != '')
"ExclusiveStartKey": {
"Name": {
"S": "$Name"
},
"Date": {
"S": "$Date"
}
},
#end
#if($Date != '')
"FilterExpression": "begins_with(#dt, :tdt)",
"ExpressionAttributeNames": {
"#dt": "Date"
},
"ExpressionAttributeValues": {
":tdt": {
"S": "$Date"
}
}
},
#end
"TableName": "MyTable"
}
This makes the filter optional, only if Date is passed as argument