DV Bulk Editor: Bid Strategy Edit Wont Allow Change of Bid Strategy/Type for Line Items

[hgrias]

In the context of DV360 Bidding Strategies, the following editable values in the sheet/UI are mutually exclusive:

1. Fixed Bid
2. Bid Goal
3. Bid Algorithm

For example, if we wanted to change the bid type from fixed bid to bid goal, we would have to delete the value in the record's Fixed Bid Edit column and add a value to the Auto Bid Goal Edit column. When this happens, we get the following NoneType error as the original value for Fixed Bid Edit is no longer a float, but an empty cell.

Float(row['Fixed_Bid_Edit']) * 100000 TypeError: float() argument must be a string or a number, not 'NoneType'

This prevents the user from changing a Line Item's bid strategy from one to another.

[hgrias]

Possible fix outlined in #42 ; needs review