Funnel analysis matches *any* sequence of events if no event matches

[wlach]

A funnel analysis like this, which looks at an event with category "foo" and an event "bar" (which obviously doesn't exist) says that every single client matches the event analysis:

https://mozilla.cloud.looker.com/explore/fenix/funnel_analysis?toggle=fil&qid=5qTHsyiW6boprm8tBOg4qp

This is a bit of a gotcha when using the explore, so we should fix it.

Underneath the hood, the funnel analysis explore uses event_analysis.aggregate_match_strings, which returns an empty string '' in the case that there is no input (which obviously anything will match). The simplest fix is probably to change the funnel analysis explore to match a short regular expression that will never occur in events_daily like ^,$ for this case. I think you'd probably want to do this around here:

lookml-generator/generator/views/funnel_analysis_view.py

Line 158 in f40a825

"SELECT "