Correcting privateChannelAddEventListenerRequest to include null as a valid listenerType

[kriswest]

Describe your change

The DesktopAgent API allows null as the type argument for PrivateChannel.addEventListener but the privateChannelAddEventListenerRequest message schema intended to support that API call does not and needs to be corrected.

Related Issue

resolves #1445

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Review Checklist

• Issue: If a change was made to the FDC3 Standard, was an issue linked above?
• N/A CHANGELOG: Is a CHANGELOG.md entry included?
• API changes: Does this PR include changes to any of the FDC3 APIs (DesktopAgent, Channel, PrivateChannel, Listener, Bridging)?
  • Docs & Sources: If yes, were both documentation (/docs) and sources updated?
    
    JSDoc comments on interfaces and types should be matched to the main documentation in /docs
  • Conformance tests: If yes, are conformance test definitions (/toolbox/fdc3-conformance) still correct and complete?
    
    Conformance test definitions should cover all required aspects of an FDC3 Desktop Agent implementation, which are usually marked with a MUST keyword, and optional features (SHOULD or MAY) where the format of those features is defined
  • Schemas: If yes, were changes applied to the Bridging and FDC3 for Web protocol schemas?
    
    The Web Connection protocol and Desktop Agent Communication Protocol schemas must be able to support all necessary aspects of the Desktop Agent API, while Bridging must support those aspects necessary for Desktop Agents to communicate with each other
    • If yes, was code generation (npm run build) run and the results checked in?
      
      Generated code will be found at /src/api/BrowserTypes.ts and/or /src/bridging/BridgingTypes.ts
• Context types: Were new Context type schemas created or modified in this PR?
  • Were the field type conventions adhered to?
  • Was the BaseContext schema applied via allOf (as it is in existing types)?
  • Was a title and description provided for all properties defined in the schema?
  • Was at least one example provided?
  • Was code generation (npm run build) run and the results checked in?
    
    Generated code will be found at /src/context/ContextTypes.ts
• Intents: Were new Intents created in this PR?
  • Were the intent name prefixes and other naming conventions & characteristics adhered to?
  • Was the new intent added to the list in the Intents Overview?

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[kemerava]

@kriswest does this need a CHANGELOG?

[kriswest]

@kemerava I did ponder that. I think that as its an unreleased version thats being corrected for an unintended error, no. But opinions may vary on that.

[kemerava]

LGTM

[kriswest]

Approved at SWG #1458