null-guard defaultAvatarUrlForString

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/Avatar.js

@@ -82,6 +82,7 @@ function urlForColor(color) {
 const colorToDataURLCache = new Map();
 
 export function defaultAvatarUrlForString(s) {
+    if (!s) return "";
     const defaultColors = ['#0DBD8B', '#368bd6', '#ac3ba8'];
     let total = 0;
     for (let i = 0; i < s.length; ++i) {