fix(openai): tolerate empty/null fields in streaming tool-call deltas (#9828)

Some OpenAI-compatible providers emit empty or null name/arguments/id
fields in streaming continuation deltas to keep the response schema
stable. pgAdmin's accumulator overwrote the real tool name (captured in
the first delta) with the later null, producing a tool call named "null"
that could not be dispatched.

Skip falsy name/arguments/id when accumulating (matching the OpenAI
Python SDK, which ignores nulls the same way) so the values captured in
the first delta survive. Also guard against a null `function` object in
a delta, which previously raised TypeError. Without the id guard a null
id in a continuation delta clobbered the real id, which the final build
then replaced with a random uuid rather than the provider's id.

Adds a unit test covering the null-continuation, multi-chunk-arguments,
and null-function cases, and a 9.16 release note.