Empty XML Fallback?

Question

I'm trying to put some code that pulls in product names, but if that field is blank, I'm getting this error:

XML parameter cannot be null or an empty string.
Function Name: TransformXML
Parameter Name: XML
Parameter Ordinal: 1
Parameter Type: Text
Submitted Value: 
ClientID: 1041374
JobID: 0
ListID: 0
BatchID: 0
SubcriberID: 454254789
Data Source Type: CustomObject
Data Source ID: 38577ee7-a008-e411-9421-3c4a92f8dd8c

I tried two different ways to put a fallback for blank ones:

%%[ if @RegisteredItems == "" or @RegisteredItems is NULL THEN ]%% We have received your registration for your gear. %%[ELSE]%% We have received your registration for the following products:

%%=TransformXML(TRIM(RegisteredItems), @xsl)=%% %%[ENDIF]%%

This works for ones with the necessary data but still results in the same error on the blank ones.

%%[IF NOT EMPTY(@RegisteredItems) THEN]%% We have received your registration for the following products:<br><br />
%%=TransformXML(TRIM(RegisteredItems), @xsl)=%% %%[ELSE]%% We have received your registration for your gear. %%[ENDIF]%%

This just shows the fallback for every instance. Is there a way to get this to fully work?

Edit: Thought it might be beneficial to add in the code for the XSL itself.

<xsl:stylesheet version='1.0' 
xmlns:xsl='http://www.w3.org/1999/XSL/Transform'>  
<xsl:template match='/'>  

  <xsl:for-each select='registrations/registration'>  
  <b>Product Name:</b> <xsl:value-of select='product/name'/><br/>  
   <b>SKU:</b> <xsl:value-of select='product/sku'/><br/>  
<b>Warranty Start Date:</b> <xsl:value-of select='purchase-date'/><br/>         
<b>Warranty End Date:</b> <xsl:value-of select='warranty-end-date'/><br/>  
<br/>  
      </xsl:for-each>  
</xsl:template>  
</xsl:stylesheet>  

Answer 1

Inside your TransformXML function you are referencing "RegisteredItems" without the @, meaning it is being pulled from a data extension or profile attribute. When you are doing the null check on it though, you are referencing "@RegisteredItems", an empty AMPscript variable, therefore it is always returning the default XML because you never instantiate @RegisteredItems.

Try this:

%%[IF NOT EMPTY(RegisteredItems) THEN]%% 
We have received your registration for the following products:<br><br />
%%=TransformXML(TRIM(RegisteredItems), @xsl)=%% 
%%[ELSE THEN]%% 
We have received your registration for your gear. 
%%[ENDIF]%%