> The select expression yields a node-sequence, the as
> expression requires atomic values, so XSLT invokes
> atomization. The result of atomizing an empty node-sequence
> is an empty sequence of strings.

The confusion comes from this:

<abc/>

And a variable that selects <abc>:

<xsl:variable name="abc" select="abc" as="xs:string?"/>

And then use the test:

$abc = ''

The result is true.  However, if <abc/> is not present, the result is
false.

So, what is the difference between the atomization process when the node
<abc/> is present but empty, and when it's not there?