ManyToManyRawIdWidget.value_from_datadict is inexplicably complicated and can be simplified

[Germano Gabbianelli]

ManyToManyRawIdWidget.value_from_datadict can be simplified from:

!#python
    def value_from_datadict(self, data, files, name):
        value = data.get(name, None)
        if value and ',' in value:
            return data[name].split(',')
        if value:
            return [value]
        return None

to:

    def value_from_datadict(self, data, files, name):
        value = data.get(name)
        if value:
            return value.split(',')

[Germano Gabbianelli]

I've not tested it

[Honza Král]

(In [14283]) Fixed #14407 -- Simplified ManyToManyRawIdWidget.value_from_datadict, Thanks tyrion!