Fix function macros with empty params list (#189)

This fixes a bug in compiling function macros without params that was
present in ctypesgen since at least 2010 according to git blame.

Prior to this fix, e.g. Py_UNREACHABLE from pymacro.h would translate to
Py_UNREACHABLE = Py_FatalError("Unreachable C code path reached")
which would promptly raise an exception on import time.

ctypesgen/printer_python/printer.py

@@ -418,10 +418,12 @@ def print_variable(self, variable):
             )
 
     def print_macro(self, macro):
-        if macro.params:
-            self.print_func_macro(macro)
-        else:
+        # important: must check precisely against None because params may be
+        # an empty list for a func macro
+        if macro.params is None:
             self.print_simple_macro(macro)
+        else:
+            self.print_func_macro(macro)
 
     def print_simple_macro(self, macro):
         # The macro translator makes heroic efforts but it occasionally fails.