[3.7] bpo-33383: Fix crash in get() of the dbm.ndbm database object. (GH-6630) (GH-6631)

(cherry picked from commit 2e38cc39330bd7f3003652869b644110a97a78d8)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Modules/clinic/_dbmmodule.c.h b/Modules/clinic/_dbmmodule.c.h
index 0f831c9..574cc5e 100644
--- a/Modules/clinic/_dbmmodule.c.h
+++ b/Modules/clinic/_dbmmodule.c.h
@@ -39,7 +39,7 @@
 }
 
 PyDoc_STRVAR(_dbm_dbm_get__doc__,
-"get($self, key, default=b\'\', /)\n"
+"get($self, key, default=None, /)\n"
 "--\n"
 "\n"
 "Return the value for key if present, otherwise default.");
@@ -57,7 +57,7 @@
     PyObject *return_value = NULL;
     const char *key;
     Py_ssize_clean_t key_length;
-    PyObject *default_value = NULL;
+    PyObject *default_value = Py_None;
 
     if (!_PyArg_ParseStack(args, nargs, "s#|O:get",
         &key, &key_length, &default_value)) {
@@ -141,4 +141,4 @@
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=5c858b4080a011a4 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=1cba297bc8d7c2c2 input=a9049054013a1b77]*/