fix(ui): fix canvas color picker when value is zero

good ol' zero is false-y

invokeai/frontend/web/src/features/canvas/hooks/useColorUnderCursor.ts

@@ -37,7 +37,12 @@ const useColorPicker = () => {
           1
         ).data;
 
-      if (!(a && r && g && b)) {
+      if (
+        r === undefined ||
+        g === undefined ||
+        b === undefined ||
+        a === undefined
+      ) {
         return;
       }