I am using this code to merge one dict into another - if a key exists, it should merge the values from both dicts:

```
source_dict = defaultdict(set)
target_dict = defaultdict(set)

def merge_dict(source_dict, target_dict):
    for source_key in source_dict:
        if source_key in target_dict:
            target_dict[source_key].update(source_dict[source_key])
        else:
            target_dict[source_key] = source_dict[source_key]

```

Is there a way to optimize the `merge_dict` function above? For example, to make it more concise or more efficient?