[BUG] fix is_optional_type to not return true for all union types

[pvditt]

Tracking issue

fixes: https://linear.app/unionai/issue/COR-2132/launch-plan-w-union-type-param-breaks

Why are the changes needed?

registering a workflow fails if it contains param of type union

@task
def calculate_interest(principal: int, rate: typing.Union[float, int], time: int) -> float:
    return (principal * rate * time) / 12


@workflow
def interest_workflow(principal: int, rate: typing.Union[float, int], time: int) -> float:
    return calculate_interest(principal=principal, rate=rate, time=time)

Request rejected by the API, due to Invalid input.
RPC Failed, with Status: StatusCode.INVALID_ARGUMENT
        Details: Type mismatch for Parameter rate in default_inputs has type union_type:{variants:{simple:FLOAT  structure:{tag:"float"}}  variants:{simple:INTEGER  structure:{tag:"int"}}}, expected simple:NONE

This occurs due to: #2327 + #2724

What changes were proposed in this pull request?

fix is_optional_type to not return true for all union types

How was this patch tested?

• updated unit tests
• ran workflows in sandbox to confirm registration issues are resolved

Setup process

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Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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