Refactor PyLint "inconsistent-return-statements"

platformio/builder/tools/piolib.py

@@ -693,7 +693,7 @@ def GetLibBuilders(env):  # pylint: disable=too-many-branches
         if lb.name in env.get("LIB_IGNORE", []):
             if verbose:
                 sys.stderr.write("Ignored library %s\n" % lb.path)
-            return
+            return None
         if compat_mode > 1 and not lb.is_platforms_compatible(
                 env['PIOPLATFORM']):
             if verbose:

platformio/builder/tools/piomisc.py

@@ -50,7 +50,7 @@ class InoToCPPConverter(object):
     def convert(self, nodes):
         contents = self.merge(nodes)
         if not contents:
-            return
+            return None
         return self.process(contents)
 
     def merge(self, nodes):

platformio/builder/tools/piowinhooks.py

@@ -72,7 +72,7 @@ def exists(_):
 
 def generate(env):
     if system() != "Windows":
-        return
+        return None
 
     env.Replace(_long_sources_hook=long_sources_hook)
     env.Replace(_long_incflags_hook=long_incflags_hook)