Empty runtime config file produces exception

[tyhoff]

Issue

When running Invoke with an empty runtime config file, the following exception is hit.

$ inv --list
Traceback (most recent call last):
  File "/Users/username/miniconda2/envs/env/bin/inv", line 11, in <module>
    sys.exit(program.run())
  File "/Users/username/miniconda2/envs/env/lib/python2.7/site-packages/invoke/program.py", line 313, in run
    self.create_config()
  File "/Users/username/miniconda2/envs/env/lib/python2.7/site-packages/invoke/program.py", line 243, in create_config
    self.config = self.config_class()
  File "/Users/username/miniconda2/envs/env/lib/python2.7/site-packages/invoke/config.py", line 641, in __init__
    self.merge()
  File "/Users/username/miniconda2/envs/env/lib/python2.7/site-packages/invoke/config.py", line 927, in merge
    self._merge_file('user', "Per-user")
  File "/Users/username/miniconda2/envs/env/lib/python2.7/site-packages/invoke/config.py", line 951, in _merge_file
    merge_dicts(self._config, data)
  File "/Users/username/miniconda2/envs/env/lib/python2.7/site-packages/invoke/config.py", line 1170, in merge_dicts
    for key, value in updates.items():
AttributeError: 'NoneType' object has no attribute 'items'

Desired Result

Passing in a valid, empty .yml file should not throw an exception.

Steps to reproduce

1. Create a new directory
2. Create a simple tasks.py file

from invoke import task, Collection, Config

@task
def hello(ctx):
    print("Hello {}".format(ctx.name))

3. Create an empty ~/.invoke.yaml file
4. Run invoke --list

Fix

I think it's reasonable to just return early if the dict being passed in is None.

diff --git a/invoke/config.py b/invoke/config.py
index 6d13ac08..84501133 100644
--- a/invoke/config.py
+++ b/invoke/config.py
@@ -1174,6 +1174,9 @@ def merge_dicts(base, updates):

     .. versionadded:: 1.0
     """
+    if not updates:
+        return base
+
     # TODO: for chrissakes just make it return instead of mutating?
     for key, value in updates.items():
         # Dict values whose keys also exist in 'base' -> recurse

---

Let me know if you need any more information or want me to throw up a commit!

[bitprophet]

Feel like I ran into this myself recently too 🙃 thanks for the detailed report! I agree that it ought to just no-op instead of exploding.

[bitprophet]

Actually, this is (perhaps unsurprisingly) an old problem and so there's an existing issue already - #270! Rolling into that. I'm just gonna fix it now either way.