eclipse-tycho#166: fix false detection of 'clean-only' builds

If default goals are used Maven can be invoked without specifying goals
explicitly in the command, so the List of goals is empty.

This had the consequence, that the AbstractMavenLifecycleParticipant
false detected a clean-only build and skipped dependency/target-platform
resolution, which lead to follow up errors.

The logic to detect 'clean-only' builds is improved to consider the
described case.

Change-Id: I68b7abf61835665a648a0bc006153c4631b56ff8
Signed-off-by: Hannes Wellmann <wellmann.hannes1@gmx.net>

tycho-core/src/main/java/org/eclipse/tycho/core/maven/TychoMavenLifecycleParticipant.java

@@ -240,7 +240,8 @@ private boolean disableLifecycleParticipation(MavenSession session) {
         // command line property to disable Tycho lifecycle participant
         return "maven".equals(session.getUserProperties().get("tycho.mode"))
                 || session.getUserProperties().containsKey("m2e.version")
-                || CLEAN_PHASES.containsAll(session.getGoals());
+                // disable for 'clean-only' builds. Consider that Maven can be invoked without explicit goals, if default goals are specified
+                || (!session.getGoals().isEmpty() && CLEAN_PHASES.containsAll(session.getGoals()));
     }
 
     private void configureComponents(MavenSession session) {