Fixes bug #0013431. Tests for current_date being an empty date

Calling routines assume that Today() can be represented either by None or an empty Date instance. This code allows for the latter assumption.
Also fixes debug code, which confused %s and str.format style formatting.  The style chosen is preferred by pylint.

gramps/gen/utils/alive.py

@@ -461,9 +461,7 @@ def ancestors_too_old(person, year):
                 return (None, None, "", None)
             self.pset.add(person.handle)
             LOG.debug(
-                "ancestors_too_old('%s', %s)".format(
-                    name_displayer.display(person), year
-                )
+                "ancestors_too_old('%s', %s)", name_displayer.display(person), year
             )
             family_handle = person.get_main_parents_family_handle()
             if family_handle:
@@ -669,15 +667,14 @@ def probably_alive(
     birth, death, explain, relative = probably_alive_range(
         person, db, max_sib_age_diff, max_age_prob_alive, avg_generation_gap
     )
-    if current_date is None:
+    if current_date is None or not current_date.is_valid():
         current_date = Today()
     LOG.debug(
-        "%s: b.%s, d.%s - %s".format(
-            " ".join(person.get_primary_name().get_text_data_list()),
-            birth,
-            death,
-            explain,
-        )
+        "%s: b.%s, d.%s - %s",
+        name_displayer.display(person),
+        birth,
+        death,
+        explain,
     )
     if not birth or not death:
         # no evidence, must consider alive