coreaudio-encoder: Allow fetching source properties without source

Per OBS API documentation, `get_properties` can be used to get the
properties of a source, but also of a source type. The latter would
pass a NULL pointer for a given source.

This adds the necessary change to avoid crashing OBS by passing such
a null pointer.

plugins/coreaudio-encoder/encoder.cpp

@@ -1361,24 +1361,28 @@ static bool samplerate_updated(obs_properties_t *props, obs_property_t *prop,
 
 static obs_properties_t *aac_properties(void *data)
 {
-	ca_encoder *ca = static_cast<ca_encoder *>(data);
 
 	obs_properties_t *props = obs_properties_create();
 
-	obs_property_t *p = obs_properties_add_list(
+	obs_property_t *sample_rates = obs_properties_add_list(
 		props, "samplerate", obs_module_text("OutputSamplerate"),
 		OBS_COMBO_TYPE_LIST, OBS_COMBO_FORMAT_INT);
-	add_samplerates(p, ca);
-	obs_property_set_modified_callback(p, samplerate_updated);
 
-	p = obs_properties_add_list(props, "bitrate",
-				    obs_module_text("Bitrate"),
-				    OBS_COMBO_TYPE_LIST, OBS_COMBO_FORMAT_INT);
-	add_bitrates(p, ca);
+	obs_property_set_modified_callback(sample_rates, samplerate_updated);
+
+	obs_property_t *bit_rates = obs_properties_add_list(
+		props, "bitrate", obs_module_text("Bitrate"),
+		OBS_COMBO_TYPE_LIST, OBS_COMBO_FORMAT_INT);
 
 	obs_properties_add_bool(props, "allow he-aac",
 				obs_module_text("AllowHEAAC"));
 
+	if (data) {
+		ca_encoder *ca = static_cast<ca_encoder *>(data);
+		add_samplerates(sample_rates, ca);
+		add_bitrates(bit_rates, ca);
+	}
+
 	return props;
 }