[test] Fix append_path in the empty case

[fjricci]

Summary

normpath() was being called on an empty string and appended to
the environment variable in the case where the environment variable
was unset. This led to ":." being appended to the path, since
normpath() of an empty string is '.', presumably to represent cwd.

[fjricci]

fix append_path

utils/lit/lit/llvm/config.py

def with_environment(self, variable, value, append_path = False):
             # of them.
             paths_to_add = value
             if lit.util.is_string(paths_to_add):
                 paths_to_add = [paths_to_add]
 
             def norm(x):
                 return os.path.normcase(os.path.normpath(x))
 
-            current_paths = self.config.environment.get(variable, "")
+            current_paths = self.config.environment.get(variable, None)
+            if current_paths:
                 current_paths = current_paths.split(os.path.pathsep)
                 paths = [norm(p) for p in current_paths]
+            else:
+                paths = []
+
             # If we are passed a list [a b c], then iterating this list forwards
             # and adding each to the beginning would result in b c a.  So we
             # need to iterate in reverse to end up with the original ordering.
             for p in reversed(paths_to_add):
                 # Move it to the front if it already exists, otherwise insert it at the
                 # beginning.
                 p = norm(p)
                 try: