POM: be defensive if metadata is missing

When comparing POMs, apparently it is possible for groupId,
artifactId and/or version to be missing/null. See:

https://forum.image.sc/t/special-characters-no-longer-allowed-in-script-parameter-declaration/25244/3

Fortunately, Java 8 has a nice functional syntax to help with this.

Author: ctrueden

src/main/java/org/scijava/util/POM.java

@@ -184,18 +184,19 @@ public String getCIManagementURL() {
 
 	// -- Comparable methods --
 
-	@Override
-	public int compareTo(final POM pom) {
+	private static final Comparator<String> STRING_COMPARATOR = //
+		Comparator.nullsFirst(String::compareTo);
+	private static final Comparator<POM> POM_COMPARATOR = Comparator//
 		// sort by groupId first
-		final int gid = getGroupId().compareTo(pom.getGroupId());
-		if (gid != 0) return gid;
-
+		.comparing(POM::getGroupId, STRING_COMPARATOR)
 		// sort by artifactId second
-		final int aid = getArtifactId().compareTo(pom.getArtifactId());
-		if (aid != 0) return aid;
-
+		.thenComparing(POM::getArtifactId, STRING_COMPARATOR)//
 		// finally, sort by version
-		return compareVersions(getVersion(), pom.getVersion());
+		.thenComparing(POM::getVersion, POM::compareVersions);
+
+	@Override
+	public int compareTo(final POM pom) {
+		return POM_COMPARATOR.compare(this, pom);
 	}
 
 	// -- Versioned methods --