`use-walrus-if` should handle `if not...` cases

This code is changed correctly:
```
val = do_something()
if val:
    do_something_else(val)
```

but this is not changed at this time:
```
val = do_something()
if not val:
    do_something_else(val)
```
 and should instead be:
```
if not (val := do_something()):
    do_something_else(val)
```