provider: Implement default methods

Simply return NULL when these methods are called.

If not implemented, calling these methods may result
in crash when the subclasses haven't implemented them.

src/calls-provider.c

@@ -52,6 +52,24 @@ enum {
 };
 static GParamSpec *props[PROP_LAST_PROP];
 
+static const char *
+calls_provider_real_get_name (CallsProvider *self)
+{
+  return NULL;
+}
+
+static const char *
+calls_provider_real_get_status (CallsProvider *self)
+{
+  return NULL;
+}
+
+GListModel *
+calls_provider_real_get_origins (CallsProvider *self)
+{
+  return NULL;
+}
+
 
 static void
 calls_provider_get_property (GObject    *object,
@@ -79,6 +97,10 @@ calls_provider_class_init (CallsProviderClass *klass)
 
   object_class->get_property = calls_provider_get_property;
 
+  klass->get_name = calls_provider_real_get_name;
+  klass->get_status = calls_provider_real_get_status;
+  klass->get_origins = calls_provider_real_get_origins;
+
   props[PROP_STATUS] =
     g_param_spec_string ("status",
                          "Status",