[SYNPY-1708] None values don't become annotations

[andrewelamb]

Problem:

Setting an annotation key to None (or to a value that contains no real data) did not behave intuitively when storing an entity:

• A key set to None was stored as the literal string "None" rather than being treated as "no value".
• A list containing only None values (e.g. [None, None]) was stored as ["None", "None"].
• A list mixing None with real values (e.g. [None, "value"]) was stored as ["None", "value"], polluting the data with a bogus "None" element.

Solution:

_convert_to_annotations_list now treats None as the absence of a value:

• A value of None is skipped entirely (the key is omitted), rather than being stringified.
• None elements are stripped from list values before type inference, so a mixed list like [None, "value"] is stored as ["value"].
• A value that resolves to an empty list after stripping None (including None, [], and [None, None]) is omitted.

Type inference now runs on the None-stripped element list, so the inferred Synapse type (STRING/BOOLEAN/LONG/DOUBLE/TIMESTAMP_MS) is no longer skewed by None placeholders. The literal string "None" is unaffected and is still stored as a normal string value.

The function also gained a full docstring documenting the conversion behavior, including the heterogeneous-list fallback to STRING.

Testing:

• Added integration test test_setting_annotation_key_to_none in tests/integration/synapseclient/models/async/test_file_async.py that stores a file with one annotation set to a primitive None, one to a list of None values, one to an empty list, one to a list mixing None with a real value, and one to the literal string "None", then retrieves a fresh copy and asserts:
  • None, list-of-None, and empty-list keys are not stored at all.
  • The mixed list [None, "None"] round-trips as ["None"] (the None stripped).
  • The literal string "None" is stored as a normal string value.
• Added unit tests in tests/unit/synapseclient/unit_test_annotations.py covering _convert_to_annotations_list directly:
  • test__convert_to_annotations_list__none_values — asserts a primitive None, a list of only None ([None, None]), and an empty list are all omitted; a mixed list ([None, "value", None]) has its Nones stripped to ["value"]; and the literal string "None" is kept as a normal STRING value.
  • test__convert_to_annotations_list__mixed_list_preserves_type — asserts that stripping None from typed lists still infers the correct Synapse type (LONG, DOUBLE, BOOLEAN) rather than falling back to STRING.

[linglp]

Hi @andrewelamb ! I could approve too. I found some edge cases of null values that we could consider to cover, but I am also fine with how you have written here. There's also a small piece of dead code that can be removed.