Check if a string is null or empty in XSLT

How can I check if a value is null or empty with XSL?

For example, if categoryName is empty? I'm using a when choosing construct.

For example:

<xsl:choose>
    <xsl:when test="categoryName !=null">
        <xsl:value-of select="categoryName " />
    </xsl:when>
    <xsl:otherwise>
        <xsl:value-of select="other" />
    </xsl:otherwise>
</xsl:choose>

---

test="categoryName != ''"

Edit : This covers the most likely interpretation, in my opinion, of "[not] null or empty" as inferred from the question, including it's pseudo-code and my own early experience with XSLT. Ie, "What is the equivalent of the following Java?":

!(categoryName == null || categoryName.equals(""))

For more details eg, distinctly identifying null vs. empty, see johnvey's answer below and/or the XSLT 'fiddle' I've adapted from that answer, which includes the option in Michael Kay's comment as well as the sixth possible interpretation.

---

Absent of any other information, I'll assume the following XML:

<group>
    <item>
        <id>item 1</id>
        <CategoryName>blue</CategoryName>
    </item>
    <item>
        <id>item 2</id>
        <CategoryName></CategoryName>
    </item>
    <item>
        <id>item 3</id>
    </item>
    ...
</group>

A sample use case would look like:

<xsl:for-each select="/group/item">
    <xsl:if test="CategoryName">
        <!-- will be instantiated for item #1 and item #2 -->
    </xsl:if>
    <xsl:if test="not(CategoryName)">
        <!-- will be instantiated for item #3 -->
    </xsl:if>
    <xsl:if test="CategoryName != ''">
        <!-- will be instantiated for item #1 -->
    </xsl:if>
    <xsl:if test="CategoryName = ''">
        <!-- will be instantiated for item #2 -->
    </xsl:if>
</xsl:for-each>

---

From Empty Element:

To test if the value of a certain node is empty

It depends on what you mean by empty.

Contains no child nodes: not(node())

Contains no text content: not(string(.))

Contains no text other than whitespace: not(normalize-space(.))

Contains nothing except comments: not(node()[not(self::comment())])

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