Cleanup: Clang tidy

Addressed almost all warnings except for replacing defines
with enums and variable assignment in if statements.

source/blender/freestyle/intern/python/BPy_ContextFunctions.cpp

@@ -284,10 +284,10 @@ static PyModuleDef module_definition = {
     /*m_doc*/ module_docstring,
     /*m_size*/ -1,
     /*m_methods*/ module_functions,
-    /*m_slots*/ NULL,
-    /*m_traverse*/ NULL,
-    /*m_clear*/ NULL,
-    /*m_free*/ NULL,
+    /*m_slots*/ nullptr,
+    /*m_traverse*/ nullptr,
+    /*m_clear*/ nullptr,
+    /*m_free*/ nullptr,
 };
 
 //------------------- MODULE INITIALIZATION --------------------------------