Fix CMake error when using HIP.

[joaander]

Description

Call find_package_handle_standard_args only when there are required libraries found.

Motivation and context

Prior to this change, CMake reports an error that the REQUIRED_VARS argument is missing. Nowhere does this script set REQUIRED_HIP_LIB_VARS, so ${REQUIRED_HIP_LIB_VARS} is the empty string.

How has this been tested?

This branch builds on Frontier.

Change log

Fixed:

* CMake error when ``HOOMD_GPU_PLATFORM=HIP``.

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.