Don't evaluate lambdas that return falsy values as an empty string.

Issue: If I create a Mustache lambda to return numbers and I expect 0 to be of
those numbers, I'll instead only receive an empty string for 0 because it
evaluates to falsy.

Fix: Check the result of the lambda to be undefined or null. Only then should
it return the empty string.

mustache.js

@@ -254,7 +254,8 @@ var Mustache;
         return self.render(template, context);
       };
 
-      return value.call(context.view, text, scopedRender) || "";
+      var result = value.call(context.view, text, scopedRender);
+      return result != null ? result : "";
     default:
       if (value) {
         return callback(this, context);

test/_files/check_falsy.js

@@ -0,0 +1,7 @@
+({
+  number: function(text, render) {
+    return function(text, render) {
+      return +render(text);
+    }
+  }
+})

test/_files/check_falsy.mustache

@@ -0,0 +1 @@
+<p>{{#number}}0{{/number}}</p>

test/_files/check_falsy.txt

@@ -0,0 +1 @@
+<p>0</p>