group_identify drops null group fields

Patch changes

• 100f993 group_identify() now validates the group identity before enqueuing. Previously group_identify("company", None) (or an empty-string group_type / group_key) sent a $groupidentify event with a null/empty $group_type or $group_key, which cannot address a group profile and just adds an unusable event to the project. Missing values are now dropped with a warning instead, matching the sdk-specs group-identify contract. Valid values, including non-string group keys, are passed through unchanged. — Thanks @posthog[bot]!