Summary:
previously this supposedly was

`#2  0x00007fefe9ef79e8 in ?? () at /usr/lib/dri/i965_dri.so`

nowadays the ?? is simply empty. the rating mechanism wasn't properly
handling this. treat an empty function name to mean the same as ?? and
rate the line as MissingFunction/Everything depending on whether
libraryName is also missing.

also update test expectations accordingly (could have actually seen that
discrepancy if one had paid attention to the outcoming rating :S)

CCBUG: 418538

Test Plan:
test passes. most lines from 417435 now rate `MissingFunction` and the whole trace is considered useless

```
Rating: 128 out of 364 Usefulness: Useless
90%: 327.6 70%: 254.8 40%: 145.6
```

Reviewers: cfeck, ngraham

Reviewed By: ngraham

Subscribers: ahmadsamir, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28027