pulseaudio: if source and sink are not installed, use null sink so pulseaudio will always start

instfiles/pulse/default.pa

@@ -1,7 +1,12 @@
 .nofail
 .fail
 load-module module-augment-properties
+load-module module-always-sink
+.ifexists module-xrdp-sink.so
 load-module module-xrdp-sink
+.endif
+.ifexists module-xrdp-source.so
 load-module module-xrdp-source
-load-module module-always-sink
+.endif
 load-module module-native-protocol-unix
+