JSON deserialization fails in binding expression for EventHubTrigger with cardinality set to "many"

## Context ##

I'm using a binding expression in an output-binding (in my case blob") that relies on JSON deserialisation (https://docs.microsoft.com/en-us/azure/azure-functions/functions-bindings-expressions-patterns#json-payloads).

## Works as documented for `cardinality: one` ##

#### **`function.json`**
```json
{
"scriptFile": "__init__.py",
"bindings": [
{
"type": "eventHubTrigger",
"name": "source",
"direction": "in",
"eventHubName": "",
"connection": "CN_CONNECTION_STRING",
"cardinality": "one",
"consumerGroup": "$Default"
},
{
"type": "blob",
"name": "sink",
"path": "/{resource}",
"connection": "CONTAINER_CONNECTION_STRING",
"direction": "out"
}
]
}
```

#### **`__init__.py`**
```python
import azure.functions as func

def main(source: func.EventHubEvent, sink: func.Out[str]):
body = source.get_body()
sink.set(body)
```

#### **payload**
```json
{ "resource": "my_resource" }
```

This works as documented: An output blob is created with the name set to whatever the value is for `resource` in the payload.

## Breaks for `cardinality: many` ##

The problem arises when I change cardinality to "many" (for performance reasons):

#### **`function.json`**
```json
{
"scriptFile": "__init__.py",
"bindings": [
{
"type": "eventHubTrigger",
"name": "source",
"direction": "in",
"eventHubName": "",
"connection": "CN_CONNECTION_STRING",
"cardinality": "many",
"consumerGroup": "$Default"
},
{
"type": "blob",
"name": "sink",
"path": "/{resource}",
"connection": "CONTAINER_CONNECTION_STRING",
"direction": "out"
}
]
}
```

#### **`__init__.py`**
```python
import azure.functions as func
from typing import List

def main(source: List[func.EventHubEvent], sink: func.Out[str]):
for event in source:
body = event.get_body()
sink.set(body)
```

#### **payload**
_Same as above_

When the function is invoked, and exception is raised: `No value for named parameter 'resource'`.

## Expected ##
I expected an output blob to be created for every message in the batched payload.