Filter out empty extensions when parsing PATHEXT on Windows (#1528)

Signed-off-by: Erwan Leroy <herronelou@gmail.com>

src/rez/utils/which.py

@@ -53,7 +53,7 @@ def _access_check(fn, mode):
 
         # PATHEXT is necessary to check on Windows
         pathext = env.get("PATHEXT", _default_pathext).split(os.pathsep)
-        pathext = [x.lower() for x in pathext]
+        pathext = [x.lower() for x in pathext if x]
 
     # iterate over paths
     seen = set()