support NULL as default argument in auto doc transform

Cython/Compiler/AutoDocTransforms.py

@@ -51,6 +51,8 @@ class EmbedSignature(CythonTransform):
         default_val = arg.default
         if not default_val:
             return None
+        if isinstance(default_val, ExprNodes.NullNode):
+            return 'NULL'
         try:
             denv = self.denv  # XXX
             ctval = default_val.compile_time_value(self.denv)

tests/run/embedsignatures.pyx

@@ -199,6 +199,9 @@ __doc__ = ur"""
 
     >>> print(funcdoc(f_defexpr5))
     f_defexpr5(int x=4)
+
+    >>> print(funcdoc(f_charptr_null))
+    f_charptr_null(char *s=NULL) -> char *
 """
 
 cdef class Ext:
@@ -403,6 +406,10 @@ cpdef f_defexpr4(int x = (Ext.CONST1 + FLAG1) * Ext.CONST2):
 cpdef f_defexpr5(int x = 2+2):
     pass
 
+cpdef (char*) f_charptr_null(char* s=NULL):
+    return s or b'abc'
+
+
 # no signatures for lambda functions
 lambda_foo = lambda x: 10
 lambda_bar = lambda x: 20