Fixing Analysis issue in Storage (#3191)

Fixes:

in firebase-ios-sdk/Firebase/Storage/FIRStorageListTask.m:73
Logic error: Converting a pointer value of type 'NSNumber *' to a primitive boolean value; instead, either compare the pointer to nil or call -boolValue

Author: schmidt-sebastian

Firebase/Storage/FIRStorageListTask.m

@@ -70,7 +70,7 @@ - (void)enqueue {
     // listAll() doesn't set a pageSize as this allows Firebase Storage to determine how many items
     // to return per page. This removes the need to backfill results if Firebase Storage filters
     // objects that are considered invalid (such as items with two consecutive slashes).
-    if (strongSelf->_pageSize) {
+    if (strongSelf->_pageSize != nil) {
       queryParams[@"maxResults"] = [strongSelf->_pageSize stringValue];
     }
 
@@ -124,4 +124,4 @@ - (void)enqueue {
   }];
 }
 
-@end
+@end