fix: ensure max_connection can accept None, Null, empty string (stac-utils#519)

**Description:**

The validator now converts empty strings and null-like values to None,
preventing Pydantic validation errors during application startup.

**PR Checklist:**

- [x] Code is formatted and linted (run `pre-commit run --all-files`)
- [x] Tests pass (run `make test`)
- [ ] Documentation has been updated to reflect changes, if applicable
- [x] Changes are added to the changelog

---------

Co-authored-by: Yuri Zmytrakov <yzmytrakovNB@yzmytrakovNB.local>

Author: YuriZmytrakov

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Added
 
+- Added validator for `REDIS_MAX_CONNECTIONS` to handle empty or null-like values ("", "null", None) and return None instead. [#519](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/519)
+
 ### Changed
 
 ### Fixed

stac_fastapi/core/stac_fastapi/core/redis_utils.py

@@ -36,6 +36,14 @@ def validate_db_sentinel(cls, v: int) -> int:
             raise ValueError("REDIS_DB must be a positive integer")
         return v
 
+    @field_validator("REDIS_MAX_CONNECTIONS", mode="before")
+    @classmethod
+    def validate_max_connections(cls, v):
+        """Handle empty/None values for REDIS_MAX_CONNECTIONS."""
+        if v in ["", "null", "Null", "NULL", "none", "None", "NONE", None]:
+            return None
+        return v
+
     @field_validator("REDIS_SELF_LINK_TTL")
     @classmethod
     def validate_self_link_ttl_sentinel(cls, v: int) -> int:
@@ -118,6 +126,14 @@ def validate_db_standalone(cls, v: int) -> int:
             raise ValueError("REDIS_DB must be a positive integer")
         return v
 
+    @field_validator("REDIS_MAX_CONNECTIONS", mode="before")
+    @classmethod
+    def validate_max_connections(cls, v):
+        """Handle empty/None values for REDIS_MAX_CONNECTIONS."""
+        if v in ["", "null", "Null", "NULL", "none", "None", "NONE", None]:
+            return None
+        return v
+
     @field_validator("REDIS_SELF_LINK_TTL")
     @classmethod
     def validate_self_link_ttl_standalone(cls, v: int) -> int: