(aws-sam): cannot generate string policies for CfnFunction

The property CfnFunctionProps.policies accepts a list of both objects and strings. However, when I try to generate a list with a string, an empty object is generated instead.

Reproduction Steps

Running cdk synth with the following construct:

new CfnFunction(scope, 'MyFunction', {
  policies: ['AWSLambdaExecute']
});

What did you expect to happen?

  MyFunction:
    Type: AWS::Serverless::Function
    Properties:
      Policies:
        - AWSLambdaExecute

What actually happened?

  MyFunction:
    Type: AWS::Serverless::Function
    Properties:
      Policies:
        - {}

Environment

• CDK CLI Version : 1.80.0
• Framework Version:
• Node.js Version: 12.18.3
• OS : Ubuntu 19.10
• Language (Version):

Other

Note that if I use a string, instead of a list of strings, the output is correct.

new CfnFunction(scope, 'MyFunction', {
  policies: 'AWSLambdaExecute'
});

  MyFunction:
    Type: AWS::Serverless::Function
    Properties:
      Policies: 'AWSLambdaExecute'

---

This is 🐛 Bug Report

[skinny85]

Which means we need to make sure that a string is not a valid object in our validation logic.

[github-actions[bot]]

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