Fix future sketchy-null errors after bug fix in '@flow strict' files

Reviewed By: wcheng86

Differential Revision: D9599872

fbshipit-source-id: dabcfd6fb7b6ec5a13801e84fe55d7d3c7ba9adb

Author: gkz

server/util/debugger-ui/deltaUrlToBlobUrl.js

@@ -41,12 +41,16 @@
 
     // If nothing changed, avoid recreating a bundle blob by reusing the
     // previous one.
-    if (deltaPatcher.getLastNumModifiedFiles() === 0 && cachedBundle) {
+    if (
+      deltaPatcher.getLastNumModifiedFiles() === 0 &&
+      cachedBundle != null &&
+      cachedBundle !== ''
+    ) {
       return cachedBundle;
     }
 
     // Clean up the previous bundle URL to not leak memory.
-    if (cachedBundle) {
+    if (cachedBundle != null && cachedBundle !== '') {
       URL.revokeObjectURL(cachedBundle);
     }