CMake: More Robust git STRIP

[ax3l]

Sometimes a git executable can be found but is unusable.
Quoting the result for empty strings makes the build logic more robust in such scenarios.

Mitigates:

CMake Error at cmake/HiPACEFunctions.cmake:192 (string):
  string sub-command STRIP requires two arguments.
Call Stack (most recent call first):
  CMakeLists.txt:256 (get_source_version)

Provoked via: Mixing of brew and Spack (git from brew + Spack env with export DYLD_LIBRARY_PATH=/Users/axel/src/spack/var/spack/environments/warpx-dev/.spack-env/view/lib)

Same as ECP-WarpX/WarpX#2358

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[SeverinDiederichs]

Great, thanks!