fix: #382 prevent network calls for the falsy/undefined src values inside isImageValid (#383)

Author: varunsharma27

src/util.ts

@@ -1,5 +1,9 @@
 export function isImageValid(src: string) {
   return new Promise(resolve => {
+    if (!src) {
+      resolve(false);
+      return;
+    }
     const img = document.createElement('img');
     img.onerror = () => resolve(false);
     img.onload = () => resolve(true);