Fix NoneType crash in premium sensors and cart service calls

[TeTeHacko]

Problem

Two related bugs cause crashes for users whose Rohlík API returns `"premiumLimits": null`
(users without Premium or with certain membership tiers):

Bug 1: `premiumLimits: null` crashes sensor and binary_sensor platforms

`dict.get(key, default)` only returns the default when the key is missing.
When the API explicitly returns `"premiumLimits": null`, `.get('premiumLimits', {})`
returns `None`, and the subsequent `.get('ordersWithoutPriceLimit', ...)` raises:

AttributeError: 'NoneType' object has no attribute 'get'

This prevents `binary_sensor.tomas_herceg_premium_membership` and
`sensor.tomas_herceg_no_limit_orders` / `sensor.tomas_herceg_free_express_deliveries`
from being added to HA at all.

Traceback from HA log:

ERROR homeassistant.components.binary_sensor] Error adding entity
binary_sensor.xxx_premium_membership for domain binary_sensor with platform rohlikcz
  File "binary_sensor.py", line 138, in extra_state_attributes

Bug 2: `add_to_cart` / `delete_from_cart` service calls return HTTP 500

Because these methods call `await self.async_update()` after the API call, which
triggers `publish_updates()` → sensor callbacks → the broken sensors above →
the exception propagates and the entire service call fails with:

HomeAssistantError: Failed to search for product: 'NoneType' object has no attribute 'get'

This means `search_and_add_to_cart` (and `add_to_cart`, `delete_from_cart`) always
return 500 for affected users, even though the underlying API call succeeded.

Traceback:

ERROR [custom_components.rohlikcz.services] Failed to search for product:
'NoneType' object has no attribute 'get'
  File "services.py", line 81, in async_search_and_add_product_service
  File "hub.py", line 99, in search_and_add
  File "hub.py", line 75, in add_to_cart
  File "hub.py", line 55, in async_update
  File "hub.py", line 68, in publish_updates
  File "binary_sensor.py", line 138, in extra_state_attributes

Fix

1. `binary_sensor.py` / `sensor.py`: Use `or {}` instead of `, {}` default
   to handle explicit `null` values from the API.

2. `hub.py`: Remove `await self.async_update()` from `add_to_cart` and
   `delete_from_cart`. The periodic update via `UpdateSensor` (every 10 min)
   is sufficient; triggering a full refresh as a side-effect of a service call
   is what causes the cascade failure.

Tested on

• HA 2025.x, rohlikcz v0.4.0
• User account without Rohlík Premium (API returns `"premiumLimits": null`)