GitRepository: slightly change handling of extra_env arg

No need to build 'env' arg for popen when extra_env is empty. In some
methods an empty dict is used to indicate "no extra env". This commit
does not affect the output/outcome of GitRepository operations, but,
just makes the code a tiny bit sleeker.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>

gbp/git/repository.py

@@ -134,7 +134,7 @@ def __init__(self, path, toplevel=True):
     def __build_env(extra_env):
         """Prepare environment for subprocess calls"""
         env = None
-        if extra_env is not None:
+        if extra_env:
             env = os.environ.copy()
             env.update(extra_env)
         return env