@notaciones

Different possible meanings for 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())])


To test if the element exists:

<xsl:template match="book">
<xsl:if test="author">

To test if the element exists and is non-empty:

<xsl:template match="book">
<xsl:if test="string(author)">

To test that the element exists and has text and/or element content:

<xsl:if test="author/* or author/text()">

and even more easily but not quite equivalent as it also tests for comments and processing instructions:

<xsl:when test="node()">