fix: correct _set_value return value for IntesisHomeLocal

[kesawi]

Problem

IntesisHomeLocal._set_value() was returning None on success, causing all set commands (power, mode, fan speed, temperature, etc.) to incorrectly signal failure to callers.

The root cause is two-fold:

1. The device responds to successful SET commands with {'success': True, 'data': None} — _request() was returning json_response.get("data") which evaluates to None
2. _set_value() was passing the raw response through, so callers doing bool(await self._set_value(...)) always received False even on success

Fix

• _request() now returns {} instead of None when a successful response has no data payload, preserving the semantic that None means failure
• _set_value() now returns result is not None — explicitly True on success, False on failure

Impact

• Fixes all local HTTP set commands for IntesisHomeLocal devices
• No impact on cloud (IntesisHome) or IntesisBox — both have separate _set_value implementations
• All 59 existing tests pass

Context

This bug was discovered while testing dolkensp's hass-intesishome (jnimmo/hass-intesishome:PR #41) which introduced _expect_ack() to surface failed set commands as HA errors — the correct behaviour that exposed this pre-existing bug.