add logic to standalone gqs to handle samples where parameter is a zero-length container

[mitzimorris]

Summary:

see discussion https://discourse.mc-stan.org/t/generated-quantities-for-0-dim-vectors-matrices/11514/8 and here https://github.com/stan-dev/rstan/issues/708

Description:

before calling transform_inits, create var_context where all params are present with default value 0 - because transform_inits tries to unconstrained all parameters.

Reproducible Steps:

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Current Output:

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Expected Output:

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Additional Information:

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Current Version:

v2.21.0