Make python's get_octopusvariable return empty string if not existing

[mik-ky]

Python and bash have different behaviours with get_octopusvariable; If the variable doesn't exist, bash will return an empty string whereas python will throw an error, failing the script.

To replicate, use the following scripts for Run a Script:

# python script
print("Color from pound-braces #{Color}")
print("Color from get_octopusvariable "+get_octopusvariable("Color"))

# bash script
echo "Color from pound-braces #{Color}"
echo "Get Octopus Variable for Color: " $(get_octopusvariable "Color")

This is a simple fix to make the behaviour consistent in python. Related: #217

[sc-49511]

Before

After

[shortcut-integration]

This pull request has been linked to Shortcut Story #49511: Variable substitution in Python - Requested by Britton Riggs.