[MNG-7716] ConcurrencyDependencyGraph deadlock if no root is selected

If ConcurrencyDependencyGraph#getRootSchedulableBuilds returns an empty
list then MultiThreadedBuilder is locked forever as it never gets a
build result (because nothing is scheduled).

This changes the method, that in such case just the first project is
returned, this might not give the best performance, but ensures that
there is at least one build scheduled and the build-loop can proceed.

maven-core/src/main/java/org/apache/maven/lifecycle/internal/builder/multithreaded/ConcurrencyDependencyGraph.java

@@ -69,6 +69,10 @@ public List<MavenProject> getRootSchedulableBuilds() {
                 result.add(projectBuild.getProject());
             }
         }
+        if (result.isEmpty() && projectBuilds.size() > 0) {
+            // Must return at least one project
+            result.add(projectBuilds.get(0).getProject());
+        }
         return new ArrayList<>(result);
     }
 
@@ -93,14 +97,21 @@ private List<MavenProject> getSchedulableNewProcesses(MavenProject finishedProje
                 result.add(dependentProject);
             }
         }
+        if (result.isEmpty()) {
+            Set<MavenProject> unfinishedProjects = getUnfinishedProjects();
+            if (unfinishedProjects.size() > 0) {
+                // Must return at least one project
+                result.add(unfinishedProjects.iterator().next());
+            }
+        }
         return result;
     }
 
     /**
      * @return set of projects that have yet to be processed successfully by the build.
      */
     public Set<MavenProject> getUnfinishedProjects() {
-        Set<MavenProject> unfinished = new HashSet<>(projectBuilds.getProjects());
+        Set<MavenProject> unfinished = new LinkedHashSet<>(projectBuilds.getProjects());
         unfinished.removeAll(finishedProjects);
         return unfinished;
     }