Cancel on File->Open causes crash

After File->Open, hit Cancel.

```py
Traceback (most recent call last):
  File "psql.py", line 161, in open
    data = sql_file.SQLFile.load()
  File "PatriciaSQL/sql_file.py", line 22, in load
    with open(filename, "r") as f:
FileNotFoundError: [Errno 2] No such file or directory: ''
Aborted (core dumped)
```