Add null-check in coersion tree for tostring, to disregard real nulls

[BoedieJB]

Issue?

For one of our kafka topics (Azure Eventhub) we receive a payload where data can be delivered as
{"attribute": null}. Kafka delta ingest currently treats this as "null", when it is an attribute candidate for a string type column.

Why does this happen?

In the coercion tree, this is implicitly cast to a string when the target column for this attribute is of string type.
The logic does not check for nulls, and assumes that a proper null is the string value. This is technically wrong.
Fixed that by explicitly disregard nulls in this coercion.

Result

We simulated a test where two payloads were sent frequently, one with a proper string, one with a null value.
Here you see the effect post-change, where nulls are now treated as a proper null.