fix(grafana): accept null values/labels/annotations in webhook intake

[lshearing-lavanda]

Fixes #98.

Problem

GrafanaAlertSchema types values (and labels/annotations) as
Dict[str, Any] = Field(default_factory=dict). default_factory only applies
when the key is absent, so an explicit "values": null — which Grafana
Alerting sends for alerts with no reduced numeric values (the contact-point
Test button, and no-data/resolved transitions) — fails validation with
Input should be a valid dictionary. The intake then returns 400 and the
alert is dropped.

Fix

A mode="before" field validator on GrafanaAlertSchema coerces null -> {}
for labels, annotations and values, so these Grafana-native nulls are
accepted while every other input is untouched.

@field_validator("labels", "annotations", "values", mode="before")
@classmethod
def _null_to_empty_dict(cls, value: Any) -> Any:
    return {} if value is None else value

Test

Adds test_grafana_alert_accepts_null_values to
tests/integrations/test_grafana_integration.py, asserting a payload with
values: null normalizes without error and yields values == {}.

Notes

• Behaviour is unchanged for payloads that already send dicts (real firing
  alerts carry values like {"B": 44.2, "C": 1}).
• Discovered while wiring Grafana contact points to the IncidentRelay Grafana
  intake: the contact-point "Test" button and no-data transitions were failing
  with the 400 above.

[Aidaho12]

Hello, and thank you, but it was closed in #94. Will be released with 2.3 in a week