meson: define dummy variable for unconditional definition

Since vlc_pulse unconditionally depends on libvlc_pulse which is defined only if pulse_dep is found. So add a dummy.

modules/meson.build

@@ -157,6 +157,7 @@ ksuser_lib = cc.find_library('ksuser',
 
 # Helper libraries for modules
 # These are helper libraries used by some modules
+libvlc_pulse = dependency('', required: false)
 if pulse_dep.found()
     libvlc_pulse = library('vlc_pulse',
         files('audio_output/vlcpulse.c'),