fixed isVarUndefined when the value of variable is boolean false

[vaidkaran]

Description of the Change

When a variable of like processBills:

const variables = { processBills: { type: 'Boolean', value: false } }

with type Boolean and value false is sent to gotql, gotql interprets the value of the variable as undefined.
This is because of the way the value of the variable is checked for existence.
Currently, it's checked with !variables[varName].value
And in case the variables[varName].value is a boolean false, the expression !variables[varName].value resolves to a boolean true, which is then returned by the function isVarUndefined.

To fix this, we should check for the value being undefined simply by doing variables[varName].value === undefined

Alternate Designs

Why Should This Be here?

It fixes a bug when a variable with a boolean false value is sent to gotql

Benefits

Users will be able to use variables with a boolean false value

Possible Drawbacks

Applicable Issues

[khaosdoctor]

Hi! Thanks a lot for your help! This completely passed by me, I'll merge the PR and add a test to ensure this won't occur anymore.