Don't open the dialog if we can't propose anything to the user

Currently if not a single connector can be proposed the dialog is still
opened and disturbs the user even though there is nothing much one can
do (and most probably want to do).

This first checks if there is at least one proposal before open the
dialog when importing a project.

This relates to eclipse-m2e#1012

org.eclipse.m2e.core.ui/src/org/eclipse/m2e/core/ui/internal/project/MavenProjectConfigurator.java

@@ -35,6 +35,8 @@
 
 import org.eclipse.m2e.core.MavenPlugin;
 import org.eclipse.m2e.core.internal.IMavenConstants;
+import org.eclipse.m2e.core.internal.lifecyclemapping.discovery.ILifecycleMappingRequirement;
+import org.eclipse.m2e.core.internal.lifecyclemapping.discovery.IMavenDiscoveryProposal;
 import org.eclipse.m2e.core.internal.lifecyclemapping.discovery.LifecycleMappingDiscoveryRequest;
 import org.eclipse.m2e.core.internal.project.ProjectConfigurationManager;
 import org.eclipse.m2e.core.project.LocalProjectScanner;
@@ -88,6 +90,12 @@ public IStatus runInWorkspace(IProgressMonitor monitor) throws CoreException {
                 }
                 // Some errors were detected
                 discoverProposals(discoveryRequest, monitor);
+                Map<ILifecycleMappingRequirement, List<IMavenDiscoveryProposal>> proposals = discoveryRequest
+                    .getAllProposals();
+                if(proposals.isEmpty()) {
+                  //if we can't propose anything to the user there is actually no point in open the dialog.
+                  return Status.OK_STATUS;
+                }
                 openProposalWizard(toProcess, discoveryRequest);
             } finally {
                 this.toProcess.clear();