fix: boolean handler must not coerce None or empty-string to False

[afkerhousse]

Summary

transform_recur always moves "null" to the end of the type list so it is tried last. For a schema typed ["null", "boolean"] this means boolean is attempted first.

The boolean handler fell through to bool(data) which:

• bool(None) → False — no exception raised → returns (True, False) ✗
• bool("") → False — no exception raised → returns (True, False) ✗

In both cases the null handler is never reached, so any field typed ["null", "boolean"] whose API value is None or "" is silently written as false instead of null.

This is a silent data corruption bug: null values are replaced with false in every destination without any schema mismatch error being raised.

Reproduction

from singer import Transformer, UNIX_MILLISECONDS_INTEGER_DATETIME_PARSING

schema = {"type": ["null", "boolean"]}

with Transformer(UNIX_MILLISECONDS_INTEGER_DATETIME_PARSING) as t:
    print(t.transform(None,  schema, {}))  # returns False — should be None
    print(t.transform("",    schema, {}))  # returns False — should be None
    print(t.transform(True,  schema, {}))  # returns True  ✓
    print(t.transform(False, schema, {}))  # returns False ✓

Fix

Return (False, None) early in the boolean handler when data is None or data == "", so transform_recur falls through to the null handler which correctly returns (True, None).

Impact

Any tap that syncs a nullable boolean field from an API that returns null or "" for unset values is affected. Confirmed reproduction via tap-hubspot contacts stream after its migration to the HubSpot CRM v3 API, where the API documentation explicitly states unset properties are returned as null.

Test plan

• transform(None, {"type": ["null", "boolean"]}, {}) returns None
• transform("", {"type": ["null", "boolean"]}, {}) returns None
• transform(True, {"type": ["null", "boolean"]}, {}) returns True
• transform(False, {"type": ["null", "boolean"]}, {}) returns False
• transform("true", {"type": ["null", "boolean"]}, {}) returns True
• transform("false", {"type": ["null", "boolean"]}, {}) returns False
• transform(None, {"type": "boolean"}, {}) still raises SchemaMismatch (non-nullable field)

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