Fix telemetry API receiver: creating empty log record(s) by platform events

[jerrytfleung]

Problem

Empty log records are created when subscribing platform type in telemetry API receiver and has telemetry API receiver in logs pipeline

Investigation

1. From here, log record(s) are created before checking the type of the event. Causing telemetry API receiver creates empty log record for platform.* events

Solution

• Making sure the type is either function or extension before creating an log record.
• Added related unit test cases to verify the function.

This PR is to fix this problem.