Tighten a condition to make passing None more explicit.

Cython/Compiler/Code.py

@@ -2357,7 +2357,7 @@ class CCodeWriter(object):
     def error_goto(self, pos):
         lbl = self.funcstate.error_label
         self.funcstate.use_label(lbl)
-        if not pos:
+        if pos is None:
             return 'goto %s;' % lbl
         return "__PYX_ERR(%s, %s, %s)" % (
             self.lookup_filename(pos[0]),