Describe the bug

When an EventPattern property (such as detailType, source, account, etc.) is set to an empty array [], CDK synthesizes the template without any error. However, EventBridge rejects empty arrays in event patterns, causing a CloudFormation deployment failure:

Resource handler returned message: "Event pattern is not valid. Reason: Empty arrays are not allowed
at [Source: (String)"{"detail-type":[],"source":["example.source"],"account":["123456789012"]}";
line: 1, column: 18] (Service: EventBridge, Status Code: 400, ...)

CDK should validate that no EventPattern field is set to an empty array during synthesis, providing a clear error message before deployment.

Expected Behavior

cdk synth should throw a validation error when any EventPattern field is an empty array, e.g.:

Error: Invalid event pattern: 'detailType' must be a non-empty array

Current Behavior

CDK synthesizes the template successfully. The error is only discovered at deploy time when CloudFormation calls the EventBridge PutRule API.

Reproduction Steps

import * as events from 'aws-cdk-lib/aws-events';

const rule = new events.Rule(this, 'Rule', {
  eventPattern: {
    source: ['my.source'],
    detailType: [],   // <-- empty array, should be caught at synth time
  },
});

Running cdk synth succeeds. Running cdk deploy fails with the EventBridge validation error above.

Additional Information/Context

• The Match class already validates for non-empty collections (e.g., Match.anythingBut() throws if given an empty list), so there's precedent for this kind of validation in the same module.
• The fix would be in renderEventPattern() in packages/aws-cdk-lib/aws-events/lib/util.ts — adding a check that no array value is empty before rendering the pattern.
• All array-typed fields on the EventPattern interface should be validated: account, detailType, detail, id, region, resources, source, time, version.

CDK CLI Version

2.1104.0

Framework Version

aws-cdk-lib 2.236.0

Node.js Version

24.13.0

OS

macOS (Darwin 25.2.0)

Language

TypeScript

Language Version

5.9.3

Other information

No response