Fix JSON parse error on SSE events with empty data

[felixweinberger]

Priming events for resumability (SEP-1699) have an event ID but empty data. The client was attempting JSON.parse("") which throws "Unexpected end of JSON input".

Motivation and Context

When connecting to a server that supports SSE resumability, the server sends priming events like:

id: 123
data: 

The client's SSE handler was only checking for empty data inside the message event type check, but priming events may not have an explicit event type. This caused a JSON parse error on every connection to servers with resumability enabled.

How Has This Been Tested?

• Added unit test that sends a priming event with empty data followed by a real message
• Verified no JSON parse error occurs
• Verified resumption token callback is still invoked for both events
• All 1130 tests pass

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/sdk@1184

commit: a8818ac