macosx: don't assign NULL as pref button tooltip

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>
(cherry picked from commit bf8c4379)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/macosx/simple_prefs.m

@@ -433,7 +433,7 @@ static inline char * __config_GetLabel( vlc_object_t *p_this, const char *psz_na
 - (void)setupButton: (NSButton *)object forBoolValue: (const char *)name
 {
     [object setState: config_GetInt( p_intf, name )];
-    [object setToolTip: [NSString stringWithUTF8String: config_GetLabel( p_intf, name )]];
+    [object setToolTip: [NSString stringWithUTF8String: config_GetLabel( p_intf, name ) ?: ""]];
 }
 
 - (void)setupField:(NSTextField *)o_object forOption:(const char *)psz_option