eclipse-iplog: Require at least one project section

We need at least one project definition to dump out a reasonably
sane IP log file in XML format.

Change-Id: I5cfcd70cd98e29159014cf3dbf0433dd9c49d49c
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.iplog/src/org/eclipse/jgit/iplog/IpLogGenerator.java

@@ -211,6 +211,12 @@ public class IpLogGenerator {
 					+ " is invalid", e);
 		}
 
+		if (meta.getProjects().isEmpty()) {
+			throw new ConfigInvalidException("Configuration file "
+					+ log.getPathString() + " in commit " + commit.name()
+					+ " has no projects declared.");
+		}
+
 		for (Project p : meta.getProjects()) {
 			p.setVersion(version);
 			projects.put(p.getName(), p);