Fix exception when context value is empty string

An exception was being thrown due to the empty string '' being treated
as a non-existent context value.  When the empty string was encountered
as a nested value in the context it caused the fallback to be accessed
instead, and due to the non-existent nesting property it threw an
exception.

lib/handlebars.js

@@ -340,7 +340,7 @@ Handlebars.Compiler.prototype = {
     if (depth > 0) {
       return "( stack[stack.length - " + depth + "]" + paramExpr + ")";
     } else {
-      return "( context" + paramExpr + " || fallback" + paramExpr + " )";
+      return "( context" + paramExpr + " != null ? context" + paramExpr + " : fallback" + paramExpr + " )";
     }
   },