I don't think it's void. PIP means a s set of measurable goals within a time frame to measure your performance. HR not involved also is a way to measure your performance.So if the HR is not involved, can the PIP be considered void, and results cannot be used as grounds of dismissal?
Usually hiring manager will not unilaterally put someone on PIP without HR in the loop anyway. It's part of company sop to ensure processes are followed