ECS: Memory reservation is not optional

### Describe the bug

task memory config is required in CDK but not supported by windows instances

https://github.com/aws/aws-cdk/blob/ddb4766785e27fbd4d672a5ff31fb07c3d3d389a/packages/%40aws-cdk/aws-ecs/lib/base/task-definition.ts#L682-L695

### Expected Behavior

should allow it to be undefined

### Current Behavior

throws exception

### Reproduction Steps

leave out memory keys

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

latest

### Framework Version

_No response_

### Node.js Version

16

### OS

mac

### Language

Typescript

### Language Version

_No response_

### Other information

_No response_

Inspect packages/@aws-cdk/aws-ecs/lib/base/task-definition.ts at lines 682-695, then reproduce the exception by leaving out the memory keys. Done means the task memory configuration can be undefined for Windows instances without throwing an exception.

Written by the indexing model from the issue text.