fix defaultSrc behavior if src is null

[modernserf]

Previously, <Avatar src={null} defaultSrc={fallbackImg} /> would flicker with the fallback image, and then go blank. I believe this is because:

• if an img src is a string, it will try to load the URL at that string and error if it fails; however, if src is nullish then it just shows nothing
• useEffect runs after mount, not just after updates, so the useEffect handler immediately overwrites the initial value of activeSrc
• the previous version of the useEffect handler did not have defaultSrc as a fallback value if src was falsy, so it was setting activeSrc to src's nullish value

With this PR, <Avatar src={null} defaultSrc={fallbackImg} /> shows the fallback image consistently. I also added a story that demonstrates the defaultSrc behavior and verifies that the provided change works correctly.

[keithk]

🚀 PR was released in v0.19.0 🚀