eclipse-tycho#179 Fix false detection of 'clean-only' builds (regress…

…ion from eclipse-tycho#166)

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.

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) {