Conditional render in tagfile depending on whether the attribute is specified or not

I have a Facelet tagfile and need to render different components depending on whether the attribute is specified or not. I tried it as below,

<ui:composition xmlns="http://www.w3.org/1999/xhtml"
    xmlns:h="http://java.sun.com/jsf/html"
    xmlns:f="http://java.sun.com/jsf/core"
    xmlns:ui="http://java.sun.com/jsf/facelets"
    xmlns:p="http://primefaces.org/ui"
    xmlns:pe="http://primefaces.org/ui/extensions"
    xmlns:c="http://java.sun.com/jsp/jstl/core">

    <h:panelGrid columns="1">
        <p:outputLabel value="test1" rendered="#{empty myParam}" />
        <p:outputLabel value="test2" rendered="#{not empty myParam}" />
    </h:panelGrid>
</ui:composition>

Which is used as below,

<mt:myTag myParam="#{myManagedBean.someProperty}" />

However, it didn't work. It takes the evaluated value of #{myManagedBean.someProperty}. If it's empty, then it still shows test1. How can I check if the myParam attribute is actually being set or not?

Create another custom tag with a taghandler class which checks the presence of a certain attribute in the variable mapper of the current Facelet context, and sets a boolean in the Facelet scope indicating the presence of the desired attribute. Finally make use of it in your tagfile.

E.g.

<my:checkAttributePresent name="myParam" var="myParamPresent" />
<h:panelGrid columns="1">
    <p:outputLabel value="test1" rendered="#{not myParamPresent}" />
    <p:outputLabel value="test2" rendered="#{myParamPresent}" />
</h:panelGrid>

With this tag handler:

public class CheckAttributePresentHandler extends TagHandler {

    private String name;
    private String var;

    public CheckAttributePresentHandler(TagConfig config) {
        super(config);
        name = getRequiredAttribute("name").getValue();
        var = getRequiredAttribute("var").getValue();
    }

    @Override
    public void apply(FaceletContext context, UIComponent parent) throws IOException {
        context.setAttribute(var, context.getVariableMapper().resolveVariable(name) != null);
    }

}

Which is registered as below in your .taglib.xml:

<tag>
    <tag-name>checkAttributePresent</tag-name>
    <handler-class>com.example.CheckAttributePresentHandler</handler-class>
    <attribute>
        <name>name</name>
        <required>true</required>
        <type>java.lang.String</type>
    </attribute>
    <attribute>
        <name>var</name>
        <required>true</required>
        <type>java.lang.String</type>
    </attribute>
</tag>