How to prevent copying non-existing elements?

Lets start with the following simplified piece of XSLT code :

<xsl:template match="/">
   ....
  <moxx:someelement>
      <xsl:value-of select="/moxx:aaa/moxx:bbb"/>
  </moxx:someelement>
  ....

So when bbb is an optional element and it does NOT exist in the source XML then the following
chunk is still created in the target XML doc:

<moxx:someelement></moxx:someelement>

How can I prevent the creation of these frame element tags when there is no source element?

In pseudo code I need something like

<xsl:template match="/">
   ....
  "if /moxx:aaa/moxx:bbb exists" then (
  <moxx:someelement>
      <xsl:value-of select="/moxx:aaa/moxx:bbb"/>
  </moxx:someelement>)
  ....

How can I code this in XSLT ?

Ben