[QNN EP] Apply workaround for Conv validation bug when bias input is implicit

[adrianlizarraga]

Description

• Adds a dummy bias of all zeros when translating a Conv without an explicit bias input. This is a workaround for a QNN validation issue that fails when the optional bias input is not provided.
• Corrects logic for unpacking of non-zero int4 zero-points. Bug does not impact models because we currently only support int4 zero-points equal to 0 (symmetric quant). But this would become an issue in the future if/when QNN supports non-zero int4 zero-points (so good to fix now).

Motivation and Context

Support Conv operators without a bias input on QNN EP with the latest QNN SDK.