fix inconsistent behavior with BrushContainer + minor bug

[bees]

The inconsistent behavior is a discrepancy in how the brush container handles redrawing the brush depending on the value of the allowResize prop if defaultArea is set to 'disable'

Here's a quick demo: https://codesandbox.io/s/4jvl663pkw

This PR makes two changes, both small:

• fix a bug in a conditional check. I caught this by dumb luck, this bug is only surfaced if you defined brushDimension="x" and allowResize="false" on the brush container.
• added a check if the defaultBrushArea is defined and has area, for consistency with the behavior described in the documentation

I didn't add unit tests since there didn't seem to be any around the events for the brush helper. I feel bad submitting a PR without unit tests - I'm happy to add some if y'all have guidance on a preferred approach to testing this.

[boygirl]

@bees thank you for this change! Works well and our visual regression tests are all passing! Merging. You can expect this work to be released by Monday