issue with 'QPyNullVariant' in parsing settings

[matysek]

Someone in a comment reported the following issue:

Traceback (most recent call last):
  File "openlp/core/ui/generaltab.py", line 277, in load
    self.numberEdit.setText(settings.value(u'ccli number', u''))
TypeError: QLineEdit.setText(QString): argument 1 has unexpected type 'QPyNullVariant'

Steps to reproduce:
1. run openlp
2. Open settings
3. change something on general tab
4. save it and then you'll get the previous traceback.

Symptoms:
- OSX, latest openlp dev tree
- not sure if osx specific
- Qt 4.8.3, PyQt 4.9.4 and PyQt 4.9.6
- happens only in parsing saved QSettings - with clean install openlp starts but not the second time.
- bug appeared after the 'QString and QVariant auto-conversion' merge

Investigation:
- setting with empty value is not converted to 'None' but to type QPyNullVariant while reading.

How to fix:
- The method 'Setting.value()' should
  - convert QPyNullVariant to None if QPyNullVariant.isNull() == True
  - convert None to empty string if defaultValue type is string

Some links:
http://python.6.n6.nabble.com/QPyNullVariant-td4981824.html