[APM] Waterfall request breaks on a v1 transaction

[jasonrhodes]

When visiting a v1 transaction page, the page blows up with:

Uncaught TypeError: Cannot read property 'find' of undefined

This was originally caught by @formgeist and mentioned in #24556 - #24556 (comment)

Short-term, the fix is to short-circuit the waterfall selector entirely if it doesn't have the right data structure, but long term we should transfer the conversion we are doing in RRR's render function into a dispatched action so the data is stored the same way in v1 and v2 waterfalls.

[elasticmachine]

Pinging @elastic/apm-ui