change colorscale reset value: [] => null (for python api)

[VeraZab]

closes: #825

I tested this and it does not seem like it breaks anything for us to have the reset value, when colorscale is a variable, to be null instead of empty array.

I tested this in RCE and streambed:

• tested switching marker.color from constant to variable and changing scales, for individual and grouped traces
• tested making pie charts and changing their colors

[nicolaskruchten]

After a bunch of adding and removing of colorscales, I got to this state:

[nicolaskruchten]

Basically I no longer have the ability to choose the colorscale, and there is none. I'll try to provide a clear replication path

[nicolaskruchten]

can't replicate :(

[nicolaskruchten]

let's merge and release once this clears tests