PyCOMPS_fromxml_str: fix a crash on valid xml but no comps input

On input such as: `<whatever />` there is no `comps_doc` created ->
create it to make sure we don't crash.
This unifies the behavior with `PyCOMPS_fromxml_f`.

Also adds a unit test.

libcomps/src/python/src/pycomps.c

@@ -360,7 +360,16 @@ PyObject* PyCOMPS_fromxml_str(PyObject *self, PyObject *args, PyObject *kwds) {
     Py_CLEAR(self_comps->p_whiteout);
     COMPS_OBJECT_DESTROY(self_comps->comps_doc);
 
-    self_comps->comps_doc = parsed->comps_doc;
+    if (parsed->comps_doc) {
+        self_comps->comps_doc = parsed->comps_doc;
+    } else {
+        COMPS_Object *tmpstr;
+        tmpstr = (COMPS_Object*)comps_str("UTF-8");
+        self_comps->comps_doc = COMPS_OBJECT_CREATE(COMPS_Doc,
+                                                    (COMPS_Object*[]){tmpstr});
+        COMPS_OBJECT_DESTROY(tmpstr);
+    }
+
     COMPS_OBJECT_DESTROY(self_comps->comps_doc->log);
     self_comps->comps_doc->log = parsed->log;
     parsed->log = NULL;

libcomps/src/python/tests/__test.py

@@ -737,6 +737,13 @@ def test_xml(self):
         comps4 = libcomps.Comps()
         self.assertRaises(libcomps.ParserError, comps4.fromxml_str, str(INVALID_COMPS_XML))
 
+        VALID_XML_NO_COMPS = "<whatever />"
+        comps5 = libcomps.Comps()
+        ret = comps5.fromxml_str(str(VALID_XML_NO_COMPS))
+        # return code 1 is non fatal error
+        self.assertTrue(ret == 1, comps5.get_last_errors())
+
+
     #@unittest.skip("")
     def test_fedora(self):
         comps = libcomps.Comps()