[1.6.x] Fixed #21581 -- Fixed a number of issues with collectstatic.

When STATIC_ROOT wasn't set, collectstatic --clear would delete
every files within the current directory and its descendants.

This patch makes the following changes:

Prevent collectstatic from running if STATIC_ROOT isn't set.

Fixed an issue that prevented collectstatic from displaying the
destination directory.

Changed the warning header to notify when the command is run
in dry-run mode.

Backport of 4befb3015c from master

tests/staticfiles_tests/tests.py

@@ -223,6 +223,17 @@ class TestFindStatic(CollectionTestCase, TestDefaults):
         self.assertIn('apps', force_text(lines[1]))
 
 
+class TestConfiguration(StaticFilesTestCase):
+    def test_location_empty(self):
+        err = six.StringIO()
+        for root in ['', None]:
+            with override_settings(STATIC_ROOT=root):
+                with six.assertRaisesRegex(
+                        self, ImproperlyConfigured,
+                        'without having set the STATIC_ROOT setting to a filesystem path'):
+                    call_command('collectstatic', interactive=False, verbosity=0, stderr=err)
+
+
 class TestCollection(CollectionTestCase, TestDefaults):
     """
     Test ``collectstatic`` management command.