[dexter] Check path != None before calling os.path.exists

The test optnone-simple-functions.cpp added in D97668 fails on macOS.
os.path.exists raises an exception because we pass it None. Guard against this.

Related revision: https://reviews.llvm.org/D97668

GitOrigin-RevId: 61d314024dc447e49481cb8494bf7165f9ec1323
1 file changed