Add tests for augur curate format-dates

Shows current behavior when the date field is empty or whitespace only.

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

@@ -0,0 +1,20 @@
+Setup
+
+  $ export AUGUR="${AUGUR:-$TESTDIR/../../../../../bin/augur}"
+
+Test empty date value.
+This currently has the unexpected behavior of returning the empty string.
+
+  $ echo '{"record": 1, "date": ""}' \
+  >   | ${AUGUR} curate format-dates \
+  >     --date-fields "date"
+  {"record": 1, "date": ""}
+
+Test whitespace only date value.
+This currently raises an error.
+
+  $ echo '{"record": 1, "date": "  "}' \
+  >   | ${AUGUR} curate format-dates \
+  >     --date-fields "date"
+  ERROR: Unable to format date string '  ' in field 'date' of record 0.
+  [2]