Agent can send empty transaction and span types

[axw]

Describe the bug

StartTransaction("name", "") will yield a transaction document without a transaction type field. Similar story for StartSpan("name", ""). The UI rightfully expects a non-empty transaction type and span type.

To Reproduce
Steps to reproduce the behavior:

1. Using the agent's API, call StartTransaction("name", "").End() and StartSpan("name", "").End()
2. Wait for ~15 seconds
3. Check that the transaction and span are visible in the APM UI

Expected behavior

A default transaction/span type should be set, so the events are visible in the UI.

[felixbarny]

The server might be a better way to fix that as it then covers all agents. But fixing it on both sides doesn't hurt, either.

[simitt]

created elastic/apm-server#7376

[simitt]

from the agent spec:

If no transaction.type or span.type is provided or the value is an empty string, the agent needs to set a default value custom.