Avoid throwing a NPE in CPListElement.toString() if the classpath entry
could not be created (e.g. due to entry kind set to -1).

Change-Id: Ie6092f5dde61122bfd264de98c7486bec17b7b20
Signed-off-by: Simeon Andreev <simeon.danailov.andreev@gmail.com>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.ui/+/190208
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Reviewed-by: Jeff Johnston <jjohnstn@redhat.com>