Chunk to content on DITA Map fails when we have a key defined on the topicref

In the flowers.ditamap I have a map reference with a key defined on it:

I publish to HTML5 and it fails:

/.../plugins/org.dita.base /build_preprocess.xml:260: java.lang.NullPointerException: Cannot invoke "java.net.URI.isOpaque()" because "child" is null
at java.base/java.net.URI.resolve(URI.java:2159)
at java.base/java.net.URI.resolve(URI.java:1068)
at org.dita.dost.store.AbstractStore.getUri(AbstractStore.java:60)
at org.dita.dost.store.StreamStore.getOutputStream(StreamStore.java:362)
at org.dita.dost.writer.ChunkTopicParser.writeToContentChunk(ChunkTopicParser.java:298)
at org.dita.dost.writer.ChunkTopicParser.processChunk(ChunkTopicParser.java:261)
at org.dita.dost.writer.ChunkTopicParser.write(ChunkTopicParser.java:61)
at org.dita.dost.reader.ChunkMapReader.processCombineChunk(ChunkMapReader.java:540)
at org.dita.dost.reader.ChunkMapReader.processTopicref(ChunkMapReader.java:341)

[flowers.zip](https://github.com/dita-ot/dita-ot/files/10089891/flowers.zip)

Reproduce the HTML5 publication with the attached flowers.zip and the flowers.ditamap topicref. Start by tracing the null URI through org.dita.dost.reader.ChunkMapReader and ChunkTopicParser into org.dita.dost.store.AbstractStore and StreamStore, with build_preprocess.xml as the build entry point. Done means this keyed map reference with chunk="to-content" publishes without the NullPointerException.

Written by the indexing model from the issue text.