Correctly assign sortval = 0 when a date is EMPTY

The is_valid() method for a Date simply checks the sortval for a zero value.
However, if an empty date is passed to _calc_sort_value() it first adjusts
to 1 ALL y,m,d values that are zero.  Thus, an empty date would return a
sortval for the year 1-1-1 as a valid date.

The problematic part of the test calculates an age for somebody with a birth
date of "" (an empty string) and a death date given by a year, such as 1760.
The previous test code required it to report "greater than 110 years",
which should not be a suitable response. The previous code also reported an
age of negative 1999 years for a birth year of 2000 and a death date of the
empty string. The unit test code now tests against a reply of "unknown".

Fixes #13415, #13423.

gramps/gen/lib/date.py

@@ -1765,15 +1765,7 @@ def set(
         self.calendar = calendar
         self.dateval = value
         self.set_new_year(newyear)
-        year, month, day = self._zero_adjust_ymd(
-            value[Date._POS_YR], value[Date._POS_MON], value[Date._POS_DAY]
-        )
-
-        if year == month == day == 0:
-            self.sortval = 0
-        else:
-            func = Date._calendar_convert[calendar]
-            self.sortval = func(year, month, day)
+        self._calc_sort_value()
 
         if self.get_slash() and self.get_calendar() != Date.CAL_JULIAN:
             self.set_calendar(Date.CAL_JULIAN)
@@ -1848,14 +1840,19 @@ def _calc_sort_value(self):
         """
         Calculate the numerical sort value associated with the date.
         """
-        year, month, day = self._zero_adjust_ymd(
-            self.dateval[Date._POS_YR],
-            self.dateval[Date._POS_MON],
-            self.dateval[Date._POS_DAY],
-        )
-        if year == month == 0 and day == 0:
+        if (
+            self.dateval[Date._POS_YR]
+            == self.dateval[Date._POS_MON]
+            == self.dateval[Date._POS_DAY]
+            == 0
+        ):
             self.sortval = 0
         else:
+            year, month, day = self._zero_adjust_ymd(
+                self.dateval[Date._POS_YR],
+                self.dateval[Date._POS_MON],
+                self.dateval[Date._POS_DAY],
+            )
             func = Date._calendar_convert[self.calendar]
             self.sortval = func(year, month, day)
 

gramps/gen/lib/test/date_test.py

@@ -1055,11 +1055,11 @@ class AgeTest(BaseDateTest):
             "2000",
             "40 years",
         ),
-        ("", "1760", "greater than 110 years"),
-        ("", "1960", "greater than 110 years"),
-        ("", "2020", "greater than 110 years"),
-        ("", "3020", "greater than 110 years"),
-        ("2000", "", "(1999 years)"),
+        ("", "1760", "unknown"),
+        ("", "1960", "unknown"),
+        ("", "2020", "unknown"),
+        ("", "3020", "unknown"),
+        ("2000", "", "unknown"),
     ]
 
     def convert_to_date(self, d):