pylint: initialize lists as [], not as None

otherwise pylint makes wrong deductions

src/kajonggtest.py

@@ -342,7 +342,7 @@ def pairs(data):
 class CSV(StrMixin):
     """represent kajongg.csv"""
 
-    knownCommits = None
+    knownCommits = []
 
     def __init__(self):
         self.findKnownCommits()
@@ -368,7 +368,7 @@ class CSV(StrMixin):
     @classmethod
     def findKnownCommits(cls):
         """find known commits"""
-        if cls.knownCommits is None:
+        if not cls.knownCommits:
             cls.knownCommits = set()
             for branch in subprocess.check_output(b'git branch'.split()).decode().split('\n'):
                 if 'detached' not in branch and 'no branch' not in branch:

src/rule.py

@@ -1022,7 +1022,7 @@ class PredefinedRuleset(Ruleset):
     """special code for loading rules from program code instead of from the database"""
 
     classes = set()  # only those will be playable
-    preRulesets = None
+    preRulesets = []
 
     def __init__(self, name=None):
         Ruleset.__init__(self, name or 'general predefined ruleset')
@@ -1030,7 +1030,7 @@ class PredefinedRuleset(Ruleset):
     @staticmethod
     def rulesets():
         """a list of instances for all predefined rulesets"""
-        if PredefinedRuleset.preRulesets is None:
+        if not PredefinedRuleset.preRulesets:
             PredefinedRuleset.preRulesets = [
                 x() for x in sorted(PredefinedRuleset.classes, key=lambda x: x.__name__)]
         return PredefinedRuleset.preRulesets

src/sound.py

@@ -179,7 +179,7 @@ class Voice(StrMixin):
     destination, no tarfile is written, only the content. It makes
     only sense to cache the voice in a tarfile at source."""
 
-    __availableVoices = None
+    __availableVoices = []
     md5sumLength = 32 # magical constant
 
     def __init__(self, directory, content=None):