patch to fix datetime validation of empty string

lib-Mojolicious-Plugin-Yancy.diff

diff --git a/lib/Mojolicious/Plugin/Yancy.pm b/lib/Mojolicious/Plugin/Yancy.pm
index 7ed3e54..6ae923e 100644
--- a/lib/Mojolicious/Plugin/Yancy.pm
+++ b/lib/Mojolicious/Plugin/Yancy.pm
@@ -892,6 +892,10 @@ sub _helper_validate {
     for my $prop_name ( keys %{ $schema->{properties} } ) {
         my $prop = $schema->{properties}{ $prop_name };

+        # These blocks fix problems with validation only. If the
+        # problem is the database understanding the value, it must be
+        # fixed in the backend class.
+
         # Pre-filter booleans
         if ( is_type( $prop->{type}, 'boolean' ) && defined $item->{ $prop_name } ) {
             my $value = $item->{ $prop_name };
@@ -902,6 +906,14 @@ sub _helper_validate {
             }
             $item->{ $prop_name } = $value;
         }
+        # An empty date-time, date, or time must become undef: The empty
+        # string will never pass the format check, but properties that
+        # are allowed to be null can be validated.
+        if ( is_type( $prop->{type}, 'string' ) && $prop->{format} && $prop->{format} =~ /^(?:date-time|date|time)$/ ) {
+            if ( !$item->{ $prop_name } ) {
+                $item->{ $prop_name } = undef;
+            }
+        }
         # Always add dummy passwords to pass required checks
         if ( $prop->{format} && $prop->{format} eq 'password' && !$item->{ $prop_name } ) {
             # Add to a new copy of the item so we don't actually change