Output a string if a given element does not exist in XSLT

I have an XSLT 1.0 stylesheet that needs to either output the value of a specific element if that element exists, or output the string "NULL" if it doesn't. How can I accomplish this?

Update

The document I'm working with looks something like this:

<kanjidic2>
    <header>
        <file_version>4</file_version>
        <database_version>2010-325</database_version>
        <date_of_creation>2010-11-20</date_of_creation>
    </header>
    <character>
        <literal>亜</literal>
        <codepoint>
            <cp_value cp_type="ucs">4e9c</cp_value>
            <cp_value cp_type="jis208">16-01</cp_value>
        </codepoint>
    </character>
    <character>
        <literal>