Stop accessing None as a dictionary

PyMapping_HasKeyString does not work on None
Accidentally, it used to return false result on silent errors.

Since Python 3.13, this is what happens instead:

    >>> import pylibmc
    >>> m = pylibmc.Client(["10.0.0.1"], binary=True)
    Exception ignored in PyMapping_HasKeyString(); consider using PyMapping_HasKeyStringWithError(), PyMapping_GetOptionalItemString() or PyMapping_GetItemString():
    Traceback (most recent call last):
      File "/usr/lib64/python3.13/site-packages/pylibmc/client.py", line 142, in __init__
        super().__init__(servers=translate_server_specs(servers),
    TypeError: 'NoneType' object is not subscriptable
    ...
    Exception ignored in PyMapping_HasKeyString(); consider using PyMapping_HasKeyStringWithError(), PyMapping_GetOptionalItemString() or PyMapping_GetItemString():
    Traceback (most recent call last):
      File "/usr/lib64/python3.13/site-packages/pylibmc/client.py", line 142, in __init__
        super().__init__(servers=translate_server_specs(servers),
    TypeError: 'NoneType' object is not subscriptable

When this is run via pytest, it leads to:

    pytest.PytestUnraisableExceptionWarning: Exception ignored in PyMapping_HasKeyString(); consider using PyMapping_HasKeyStringWithError(), PyMapping_GetOptionalItemString() or PyMapping_GetItemString(): None

Fixes pallets-eco/cachelib#400

src/_pylibmcmodule.c

@@ -2082,7 +2082,7 @@ static PyObject *PylibMC_Client_set_behaviors(PylibMC_Client *self,
     char *key;
 
     for (b = PylibMC_behaviors; b->name != NULL; b++) {
-        if (!PyMapping_HasKeyString(behaviors, b->name)) {
+        if (behaviors == Py_None || !PyMapping_HasKeyString(behaviors, b->name)) {
             continue;
         } else if ((py_v = PyMapping_GetItemString(behaviors, b->name)) == NULL) {
             goto error;
@@ -2112,7 +2112,7 @@ static PyObject *PylibMC_Client_set_behaviors(PylibMC_Client *self,
     }
 
     for (b = PylibMC_callbacks; b->name != NULL; b++) {
-        if (!PyMapping_HasKeyString(behaviors, b->name)) {
+        if (behaviors == Py_None || !PyMapping_HasKeyString(behaviors, b->name)) {
             continue;
         } else if ((py_v = PyMapping_GetItemString(behaviors, b->name)) == NULL) {
             goto error;