field.process_formdata should still be called if formdata is empty

[davidism]

Some fields will set a default value if their key is not in formdata. However, process_formdata is only called if formdata is truthy. This can cause unexpected field data if formdata is empty versus if it just didn't contain the field.

class F(Form):
    name = StringField()

f = F(MultiDict())
print(f.name.data)  # None

f = F(MultiDict({'language': 'Python'}))
print(f.name.data)  # ''

This patch explicitly checks if formdata is None and adds a test for this case.

[hyperknot]

Is there no case when the block of code would handle an exception? I mean it was moved out of a try-except block, and for example if formdata.getlist(self.name) produces an exception it is now not handled.