Bug 150998 [Import/Export] Import Projects Wizard: Finish is enabled even if no project is selected
In Import Existing Projects Wizard, the Finish button is enabled even when no projects are selected for import. When the Import wizard is first invoked, Finish is disabled, since no projects have been selected for import. If a directory with no Eclipse projects is selected, Finish remains disabled. If a directory with at least one Eclipse project is selected, then Finish becomes enabled. If none of the projects are selected for import, then Finish remains enabled. Although this behaviour does not cause any functional problems, the UI seems inconsistent.
bundles/org.eclipse.ui.ide/src/org/eclipse/ui/internal/wizards/datatransfer/WizardProjectsImportPage.java
* Return null if this file does not exist or is not of valid format.
	 */
	private TarFile getSpecifiedTarSourceFile(String fileName) {
		if (fileName.length() == 0) {
			return null;
		}
		try {
			return new TarFile(fileName);
		} catch (TarException e) {
			displayErrorDialog(DataTransferMessages.TarImport_badFormat);
		} catch (IOException e) {
			displayErrorDialog(DataTransferMessages.ZipImport_couldNotRead);
		}
		archivePathField.setFocus();
		return null;
	}
	/**
	 * Display an error dialog with the specified message.
	 * 
	 * @param message
	 *            the error message
	 */
	protected void displayErrorDialog(String message) {
		MessageDialog.openError(getContainer().getShell(),
				getErrorDialogTitle(), message);
	}
	/**
	 * Get the title for an error dialog. Subclasses should override.
	 */
	protected String getErrorDialogTitle() {