With my Saxon extension function code I have the log messages:
> java -cp ./saxon-he-10.2.jar:./ net.sf.saxon.Transform -t -init:MyInitializer -xsl:./exttest.xsl -o:./out.xml -it:initialtemplate
Saxon-HE 10.2J from Saxonica
Java version 14.0.2
Stylesheet compilation time: 305.437652ms
Processing (no source document) initial template = initialtemplate
Using parser com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser
Building tree for null using class net.sf.saxon.tree.tiny.TinyBuilder
Tree built in 0.850325ms
Tree size: 3 nodes, 0 characters, 0 attributes
Execution time: 29.965658ms
Memory used: 14Mb
It is not clear to me wether Building tree for null using class net.sf.saxon.tree.tiny.TinyBuilder means that there is something wrong with my code https://gitlab.com/ms452206/socode20200915 and how to avoid it.
It's a poor message and I will improve it; the "null" would be the base URI (or systemId) of the document if it had one. The fact that the document has no known base URI could be a predictor of trouble downstream, since some things rely on a document having a known base URI; but it's not an error in itself.
It's most likely to happen if you build a document using a JAXP Source object whose systemId property is null. Which is what you have done when you wrote:
new StreamSource(new StringReader("<foo/>"))
This is likely to cause failures only if the document contains relative URIs (for example in external entity references or href or xml:base attributes), which is not the case with your simple XML document.