Fix NPE caused by response without id

[quaff]

Motivation and Context

Close GH-506

How Has This Been Tested?

I didn't find a way to write unit test due to the exception is thrown in private method and swallowed by reactor, but I observed reactor.core.Exceptions$ErrorCallbackNotImplemented: java.lang.NullPointerException: Cannot invoke "Object.hashCode()" because "key" is null is gone after this commit.

Breaking Changes

No

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

[tzolov]

Thanks @quaff,

Rebased, ad minor changes and merged at d8cc189