More clear description of missing events

[tybug]

For conditional events, the test cases where the event was not present is shown as "N/a". eg

from hypothesis import *
from hypothesis import strategies as st

@given(st.integers())
def f(n):
    if n > 100:
        event("hi")
f()

To me - and this may just be me - it wasn't clear that "N/a" means the event was not triggered. Maybe this could be made more clear for events which are either present/not present, rather than having an associated payload distribution? "Present" / "Not present" is a clear alternative to "true" / "N/a", but I'll leave the specific design to you 😄

[hgoldstein95]

I like "Present" and "Not Present"! It may render weird if one of them is a small bar, since those are kind of long labels, but I may be able to figure out a way to abbreviate them in those cases.

Feel free to make a PR if you want, but if you don't get to it I'll try to make this change the next time I'm in the code.