Add isset conditional for serialize observer

[kbjohnson90]

Without a conditional to serialize, then on update a column is created (empty), serialized, and overrides previously stored data.

[kbjohnson90]

I was having some serialized data override while trying to update another column.

If a serialized row is not set for update, then the serialize observer creates an empty column to serialize the overrides the column that is already stored in the database.