The <es:isNull> tag is used to check if a value is null. In the case of a String, the <es:isNull> tag is used to check if the String is null or is empty (zero length). An empty string is null.
<tagName attribute="value" />
item
Required (Object) - The variable to evaluate.
The following example prints a message if the value is null.
<es:isNull item="<%=value%>"> Error: the value is null.
</es:isNull>