XSL: avoid empty attributes in output stream

Whenever you create an attribute with an xsl:attribute instruction containing conditional instructions (xsl:if or xsl:choose), the result might be empty, resulting in an empty attribute in the output stream. For example, both my simplistic WordProcessingML to HTML stylesheet or similar code published on OpenXML Developer could create <span style=""> elements.

To avoid this problem, capture the conditional results into a variable and create the attribute only if the variable is non-empty, for example:

<xsl:variable name="style">
  <xsl:if test="w:rPr/w:i">font-style: italic;;</xsl:if>
  <xsl:if test="w:rPr/w:b">font-weight: bold;</xsl:if>
</xsl:variable>

<xsl:if test="$style != ''">
  <xsl:attribute name="style"><xsl:value-of select="$style" /></xsl:attribute>
</xsl:if>