small fix: hide param if no value

src/main.imba

@@ -21,7 +21,7 @@ tag view-b
 tag view-c
 	<self><h1> 'c'
 
-R.setters.e = do |v| "--{v}"
+R.setters.e = do |v| "--{v}" if v
 R.getters.e = do |v| v.slice(2)
 tag view-e
 	<self><h1> 'e'