Undefined index notice in array_merge_recursive_distinct()

`array_merge_recursive_distinct()` causes an undefined index notice, resulting in dozens of lines like the following every time I run `buildmanager-build`:
```
Undefined index: views buildmanager.drush.inc:486 [2.26 sec, 8.49MB] [notice]
```
I'm pretty sure the following is the fix. It eliminates the notices, and it shouldn't affect behavior.