fix(proxy): add safe header sanitization and null key/val guards [skip ci]

litellm/proxy/utils/header_sanitizer.py

@@ -0,0 +1,20 @@
+\"\"\"
+Header sanitization and guard utilities for LiteLLM proxy authentication forwarding.
+\"\"\"
+from typing import Dict, Any, Optional
+
+def sanitize_proxy_headers(headers: Optional[Dict[str, Any]]) -> Dict[str, str]:
+    if not headers:
+        return {}
+    
+    sanitized: Dict[str, str] = {}
+    for k, v in headers.items():
+        if k is None:
+            continue
+        key_str = str(k).strip()
+        if not key_str:
+            continue
+        val_str = "" if v is None else str(v).strip()
+        sanitized[key_str] = val_str
+        
+    return sanitized