format-dates: Return fully masked dates for empty date fields

If a date field is empty, it should be completely masked to indicate
that the date is unknown.

Resolves <#1507>

augur/curate/format_dates.py

@@ -117,6 +117,10 @@ def format_date(date_string, expected_formats):
 
 
     >>> expected_formats = ['%Y', '%Y-%m', '%Y-%m-%d', '%Y-%m-%dT%H:%M:%SZ', '%m-%d']
+    >>> format_date("", expected_formats)
+    'XXXX-XX-XX'
+    >>> format_date("  ", expected_formats)
+    'XXXX-XX-XX'
     >>> format_date("01-01", expected_formats)
     'XXXX-XX-XX'
     >>> format_date("2020", expected_formats)
@@ -133,6 +137,10 @@ def format_date(date_string, expected_formats):
     '2020-01-15'
     """
 
+    date_string = date_string.strip()
+    if date_string == '':
+        return 'XXXX-XX-XX'
+
     for date_format in expected_formats:
         try:
             parsed_date = datetime.strptime(date_string, date_format)
@@ -180,7 +188,9 @@ def run(args, records):
         for field in args.date_fields:
             date_string = record.get(field)
 
-            if not date_string:
+            # TODO: This should raise an error if the expected date field does
+            # not exist in the the record
+            if date_string is None:
                 continue
 
             formatted_date_string = format_date(date_string, args.expected_date_formats)

tests/functional/curate/cram/format-dates/empty-date-field.t

@@ -2,19 +2,16 @@ Setup
 
   $ export AUGUR="${AUGUR:-$TESTDIR/../../../../../bin/augur}"
 
-Test empty date value.
-This currently has the unexpected behavior of returning the empty string.
+Test empty date value, which should be returned as a fully masked date.
 
   $ echo '{"record": 1, "date": ""}' \
   >   | ${AUGUR} curate format-dates \
   >     --date-fields "date"
-  {"record": 1, "date": ""}
+  {"record": 1, "date": "XXXX-XX-XX"}
 
-Test whitespace only date value.
-This currently raises an error.
+Test whitespace only date value, which should be returned as a fully masked date.
 
   $ echo '{"record": 1, "date": "  "}' \
   >   | ${AUGUR} curate format-dates \
   >     --date-fields "date"
-  ERROR: Unable to format date string '  ' in field 'date' of record 0.
-  [2]
+  {"record": 1, "date": "XXXX-XX-XX"}