docs: bump sphinx from 8.1.3 to 9.0.4 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 8.1.3 to 9.0.4.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](https://github.com/sphinx-doc/sphinx/compare/v8.1.3...v9.0.4)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 9.0.4
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

docs/requirements.txt

@@ -1,2 +1,2 @@
 furo==2024.8.6
-sphinx==8.1.3
+sphinx==9.0.4

src/healthchecks_io/schemas/checks.py

@@ -128,33 +128,33 @@ class CheckCreate(BaseModel):
 
     @field_validator("schedule")
     @classmethod
-    def validate_schedule(cls, value: str | None) -> str:
+    def validate_schedule(cls, value: str) -> str:
         """Validates that the schedule is a valid cron expression."""
-        if value and not croniter.is_valid(value):
+        if not croniter.is_valid(value):
             raise ValueError("Schedule is not a valid cron expression")
         return value
 
     @field_validator("tz")
     @classmethod
-    def validate_tz(cls, value: str | None) -> str:
+    def validate_tz(cls, value: str) -> str:
         """Validates that the timezone is a valid timezone string."""
-        if value and value not in pytz.all_timezones:
+        if value not in pytz.all_timezones:
             raise ValueError("Tz is not a valid timezone")
         return value
 
     @field_validator("methods")
     @classmethod
-    def validate_methods(cls, value: str | None) -> str:
+    def validate_methods(cls, value: str) -> str:
         """Validate that methods."""
-        if value and value != "POST":
-            raise ValueError("Methods is invalid, it should be either an empty string, None, or POST")
-        return ""
+        if value not in ("", "POST"):
+            raise ValueError("Methods is invalid, it should be either an empty string or POST")
+        return value
 
     @field_validator("unique")
     @classmethod
-    def validate_unique(cls, value: List[Optional[str]] | None) -> List[Optional[str]]:
+    def validate_unique(cls, value: List[Optional[str]]) -> List[Optional[str]]:
         """Validate unique list."""
-        for unique in value or []:
+        for unique in value:
             if unique not in ("name", "tags", "timeout", "grace"):
                 raise ValueError(
                     "Unique is not valid. Unique can only be name, tags, timeout, and grace or an empty list"