Parse nulls as None when using serde-well-known

Previously when deserializing a json blob that has a field present but
set to null it would error with "invalid type: null, expected an
RFC3339-formatted `Option<OffsetDateTime>`".  When the field is present
with a value of null serde treats this as Unit.  The default
implementation always raises a type error.  This change parses it as
None as a user would expect.

src/serde/visitor.rs

@@ -291,6 +291,13 @@ impl<'a> de::Visitor<'a> for Visitor<Option<well_known::Rfc2822>> {
     fn visit_none<E: de::Error>(self) -> Result<Option<OffsetDateTime>, E> {
         Ok(None)
     }
+
+    fn visit_unit<E>(self) -> Result<Self::Value, E>
+    where
+        E: de::Error,
+    {
+        Ok(None)
+    }
 }
 
 #[cfg(feature = "serde-well-known")]
@@ -326,4 +333,11 @@ impl<'a> de::Visitor<'a> for Visitor<Option<well_known::Rfc3339>> {
     fn visit_none<E: de::Error>(self) -> Result<Option<OffsetDateTime>, E> {
         Ok(None)
     }
+
+    fn visit_unit<E>(self) -> Result<Self::Value, E>
+    where
+        E: de::Error,
+    {
+        Ok(None)
+    }
 }