icon={false} not rendering as expected for Callout component

[liaprins]

Describe the bug
Setting the icon prop for the Callout component to {false} should remove the icon, but it does not.

To Reproduce
Steps to reproduce the behavior:

1. Add icon={false} as a prop to the Callout component (e.g. in a Storybook test, where it can easily be checked)

Expected behavior
This should remove the icon, like it does for the MUI component it is built upon. I also verified with Timmy that this is the expected behavior; see more in a Zeroheight comment here.

Recommended fix (per @tihuan ), documenting here for posterity:
Change the source code from

if (icon) return icon;

to:

if (icon !== undefined) return icon;

"And then the following will work:
icon={false} - it will not take up space in the component. Similar to display: none;
icon={<></>} - it will still take up space in the component, but just empty space. Similar to visibility: hidden;
icon={null} - this is a little counter intuitive, but it will fall back to using MUI's iconMapping"
(Thanks, @tihuan !!!)

[liaprins]

PR #717 is first pass to fix this:
#717