selector arg for on(evt, selector, fn) can be false

Treat falsey values like null/undefined were treated: as if the argument
wasn't passed at all.

src/event.js

@@ -165,11 +165,11 @@
   }
 
   $.fn.on = function(event, selector, callback){
-    return selector == undefined || $.isFunction(selector) ?
+    return !selector || $.isFunction(selector) ?
       this.bind(event, selector || callback) : this.delegate(selector, event, callback)
   }
   $.fn.off = function(event, selector, callback){
-    return selector == undefined || $.isFunction(selector) ?
+    return !selector || $.isFunction(selector) ?
       this.unbind(event, selector || callback) : this.undelegate(selector, event, callback)
   }
 

test/event.html

@@ -110,10 +110,12 @@ <h1>Zepto event tests</h1>
       testOnWithNullSelector: function(t){
         var log = []
         this.el.on('click', null, function(){ log.push('a') }).
-                on('click', undefined, function(){ log.push('b') })
+                on('click', undefined, function(){ log.push('b') }).
+                on('click', false, function(){ log.push('c') }).
+                on('click', '', function(){ log.push('d') })
 
         click(this.el)
-        t.assertEqual('a b', log.sort().join(' '))
+        t.assertEqual('a b c d', log.sort().join(' '))
       },
 
       testOffWithObject: function(t){