Update WinMSSQL.py to handle default zDBInstances of "MSSQLSERVER;"

Cheap fix for the default value of zDBInstances (MSSQLSERVER;) causing the "Error parsing..." message to be thrown.  2nd iteration of instance throws the exception since arrPassword at index 1 is also an empty string.  Actually, now that I think about it, might want to change "if instance:" to "if instance and arrPassword[i]" although that is checked in the parent if statement to some degree.  Anyway, up to you guys :)

```python
>>> dbinstance = 'MSSQLSERVER;'
>>> arrInstance = dbinstance.split(';')
>>> print arrInstance
['MSSQLSERVER', '']
>>> arrPassword = 'sa:test;'
>>> dbuser, dbpass = arrPassword[1].split(':', 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 1 value to unpack
```

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