Handle empty responses from partials

Fixes #675

lib/handlebars/runtime.js

@@ -33,7 +33,7 @@ export function template(templateSpec, env) {
       // TODO : Check this for all inputs and the options handling (partial flag, etc). This feels
       // like there should be a common exec path
       var result = invokePartial.apply(this, arguments);
-      if (result) { return result; }
+      if (result != null) { return result; }
 
       var options = { helpers: helpers, partials: partials, data: data };
       partials[name] = env.compile(partial, { data: data !== undefined }, env);
@@ -42,7 +42,7 @@ export function template(templateSpec, env) {
   } else {
     invokePartialWrapper = function(partial, name /* , context, helpers, partials, data */) {
       var result = invokePartial.apply(this, arguments);
-      if (result) { return result; }
+      if (result != null) { return result; }
       throw new Exception("The partial " + name + " could not be compiled when running in runtime-only mode");
     };
   }