Fix None-valued OTel span attributes in triggerer

[jakubmatyszewski]

Noticed following error in triggerer logs:

[2026-05-05T08:02:00.644020Z] {{__init__.py:149}} ERROR - Failed to encode key result: Invalid type <class 'NoneType'> of value None
Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.13/site-packages/opentelemetry/exporter/otlp/proto/common/_internal/__init__.py", line 146, in _encode_attributes
    _encode_key_value(key, value, allow_null=allow_null)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.13/site-packages/opentelemetry/exporter/otlp/proto/common/_internal/__init__.py", line 107, in _encode_key_value
    key=key, value=_encode_value(value, allow_null=allow_null)
                   ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.13/site-packages/opentelemetry/exporter/otlp/proto/common/_internal/__init__.py", line 100, in _encode_value
    raise Exception(f"Invalid type {type(value)} of value {value}")
Exception: Invalid type <class 'NoneType'> of value None

I've compared implementation of _make_trigger_span* with _make_task_span* and my guess is that it failed due to map_index being None.
Proposed fix filters out None values before building the attributes dict.

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