DERBY-6634: Improve test coverage of SqlXmlUtil.java

Remove impossible code paths so that they don't show up as untested
code. In particular:

- Remove some unnecessary null tests. The callers already make sure
  that the arguments cannot be null, so it's better to skip the checks
  so that potential future bugs get reported as NPEs instead of being
  silently ignored.

- Remove code that serializes lists of atomic values. Derby only
  supports queries that return single atomic values.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1607017 13f79535-47bb-0310-9956-ffa450edef68