Conda: protect against None when appending core requirements
`pip` can be defined as
Ref
@@ -615,7 +615,8 @@ class Conda(PythonEnvironment):
for item in dependencies:
if isinstance(item, dict) and 'pip' in item:
- pip_requirements.extend(item.get('pip', []))
+ # NOTE: pip can be ``None``
+ pip_requirements.extend(item.get('pip') or [])
dependencies.remove(item)
break
