RationalTime shouldn't allow to set 0 as rate

## Bug Report

### Incorrect Functionality and General Questions

Describe the issue here.

## To Reproduce

1. Operating System: Any
2. Python version: Python3.9
4. OpenTimelineIO 0.14.0

```py

from opentimelineio.opentime import RationalTime, TimeRange
from opentimelineio.schema import Clip, Track, Timeline

time = RationalTime(0.0, 25)

tl = Timeline()
track = Track("Track 1")
clip = Clip("movie.mkv")
clip.source_range = TimeRange(
RationalTime(30, 0), # Must raise an error
RationalTime(10, 0), # Must raise an error
)
track.append(clip)
tl.tracks.append(track)

print(clip.source_range.duration.is_invalid_time())
# >> True

print(tl.duration())
# >> RationalTime(inf, 1)
# 'inf' is very tough to handle
```

## Expected Behavior

Setting `0` as `RationalTime.rate` value must raise a `ValueError("Rate must be positive.")`.
I'm not sure a rate could be negative though, unless it means the movie must be played reversed by the software.

Start at the opentimelineio.opentime.RationalTime entry point and reproduce the supplied Python snippet with a zero rate. Verify the existing behavior for invalid rates, then make completion mean that setting rate to 0 raises ValueError("Rate must be positive.").

Written by the indexing model from the issue text.