update expression resolver to return blank string rather than false f…

…or exception cases

src/main/java/com/hubspot/jinjava/el/ExpressionResolver.java

@@ -50,7 +50,7 @@ elContext, new StringBuilder("#{").append(cleanExpr).append('}')
           String.format("Error resolving expression: [%s]", expr), e, lineNumber)));
     }
 
-    return Boolean.FALSE;
+    return "";
   }
 
 }