Add `ensure` boolean parameter to the `ActionKeyframeStrip.channels()` RNA
function. Passing `ensure=True` will ensure the channelbag for the given
action slot handle exists. This makes it more straight-forward to create
F-Curves for a slot regardless of whether there already was a channelbag
for those F-Cuves:
```python
strip = action.layers[0].strips[0]
slot = action.slots[0]
# Old:
channelbag = strip.channels(slot.handle)
if not channelbag:
channelbag = strip.channelbags.new(action_slot)
ob_loc_x = channelbag.fcurves.new('location', index=0)
# New:
channelbag = strip.channels(slot.handle, ensure=True)
ob_loc_x = channelbag.fcurves.new('location', index=0)
```
Pull Request: https://projects.blender.org/blender/blender/pulls/133678
815337a7da