CMake: More Robust git STRIP (#2358)

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/WarpXFunctions.cmake:289 (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`)

0 files changed, 0 insertions, 0 deletions