fixing empty string check

[alexpiet]

Pull Request instructions:

• Please follow the update protocol
• Answer the questions below in detail. Your responses will be emailed to experimenters.
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Describe changes:

opto parameters throw an error when we check if the parameter can convert to float when the default is the emptystring.

What issues or discussions does this update address?

• resolves #631
• resolves #572
• resolves #191

Describe the expected change in behavior from the perspective of the experimenter

• none

Describe any manual update steps for task computers

• none

Was this update tested in 446/447?

• [ ] tested

[alexpiet]

@hanhou @XX-Yin This is ready for review